ואזנים לשמוע עד היום הזה, "and ears to listen, until this day.⁠" The message is that a student does not truly appreciate the opinions and knowledge of his teacher until he reaches the age of 40. This is why the sages in Avot 5,27 characterize the attainment of the fortieth birthday as a time when one is granted בינה, a measure of insight.

ותבואו אל המקום הזה, "and you arrived at this place.⁠" Moses inserted these two verses in the middle of his lecture as he wanted to reinforce the people's faith in the fact that they would conquer and inherit the land of Israel, just as they had conquered and inherited the lands formerly belonging to Sichon and Og who also belonged to the seven Canaanite tribes whose land had been promised to the Jewish people (Avraham and the other patriarchs). The two and a half tribes Reuven, Gad, and half the tribe of Menashe had already taken possession of these lands at the time Moses was speaking, leaving the land on the West Bank of the Jordan for the other 9 1/2 tribes.

You should appreciate that the warnings of doom enumerated in this whole portion are Moses' own wording as distinct from similar warnings of disaster in Leviticus chapter 26. This does not mean that Moses "made up the תוכחה by himself.⁠" Rather, it means that the syntax used by the Torah reflects that God approved of the choice of words Moses had used so that effectively everything recorded in the written text of the Torah is equivalent to something dictated by God Himself.
If our sages (Megillah 31) stated that the "curses contained in the Torah and which were uttered by the priests were words put in their mouths by God Himself,⁠" whereas those contained in the Book of Deuteronomy reflect Moses' words, they meant that Moses succeeded in voicing thoughts which reflected exactly what God would have told him to say.
The reason that the Torah chose to write two versions of these exhortations and warnings, one being God's initiative and words, the other at the initiative of Moses who had divined what God wanted him to say, is that the version we read at the end of the Book of Leviticus is that which became true at the time the first Temple was destroyed, whereas the longer version we read here in the Book of Deuteronomy was not fulfilled until the second Temple was being destroyed. One proof that this is so is furnished by the words (Leviticus 26,30) והשמדתי את במותיכם והכרתי את חמניכם, "I will destroy your private altars and demolish your sun-idols.⁠" These are phenomena which existed only during the period of the first Temple when idolatry was widespread among the people during the reigns of many kings. The curses listed in Deuteronomy were all fulfilled during the collapse of the second commonwealth and its Temple.

Seeing that the Shechinah resided in Jerusalem during the period of the first Temple David wrote (Psalms 24,7) "lift up your heads o gate! Up high, you everlasting doors, so the King of glory may come in!⁠" The words מלך הכבוד used by David there mean מלך של הכבוד, a reference to the Shechinah resident in Jerusalem, i.e. the Temple during that epoch. During the period of the second Temple the Shechinah did not reside permanently in Jerusalem, but merely straddled the Temple from time to time as described by Chagai 1,8: "I will look upon it (the second Temple) with favor and will be glorified, said the Lord.⁠" The letter ה at the end of the word ואכבדה in that verse is a hint that the Shechinah will be there as a loose cover, חופף. In the future, when the third Temple will be built, the Shechinah will return in full force as it had been manifest during the first Temple. In fact, its manifestation will be even more pronounced than it had been during the period of the first Temple. This is what Isaiah meant when he said (Isaiah 60, 1-2) "Arise, shine, for your light has dawned; the Presence of the Lord has shone upon you! Behold! Darkness shall cover the earth, and thick clouds the peoples, but upon you the Lord will shine, and His Presence will be over you; nations shall walk by your light, kings by your shining radiance.⁠"

טובה תוכחת מגולה מאהבה מסוטרת, "Better an open rebuke than love which is concealed.⁠" (Prov.27,5)

In his Book of Proverbs Solomon taught us important principles about the nature of rebukes and how to administer them. He taught the people that life itself is closely linked to rebukes and the ability to accept same, whereas death is inextricably involved with people who resent being rebuked and are unwilling to accept rebukes. We know that life is closely linked to rebukes from Proverbs 6,23: "instructive rebukes are a way to life.⁠" We also have a verse expressing similar sentiments in Proverbs 10,17: "he who accepts rebukes is on the way to the path of life.⁠" Yet another verse from Scripture dwelling on the same subject is found in Proverbs 15,4: "a healing tongue is a tree of life,⁠" i.e. the way to heal a tongue (which has wagged) is to rebuke the owner. On the other hand, death attaches itself to people who hate being rebuked as we know from Proverbs 15,10: "Harsh discipline is needed for those who abandon the right path; he who hates rebuke will die.⁠" He who departs from the right path (of Torah) only temporarily but does not hate being rebuked will be judged by harsh discipline in order to make him abandon the path of evil; the one who hates rebukes, however, cannot be brought back to the right path by afflictions. There is no hope for him. As a result, such a person will be sentenced to death (by the celestial court)
A love to accept rebukes is a strong indication that the person in question possesses fine qualities, generally. A negative attitude to rebukes indicates that such a person also suffers from other negative traits. This is the reason Solomon says (Proverbs 9,8) "do not rebuke a scoffer lest he will hate you, rather rebuke a wise man and he will love you.⁠" When rebuking a fellow human being one must reveal sincerity and not engage in flattery. One should give him credit for whatever good points he has, and at the same time restrict oneself to point out such a person's shortcomings, for the righteous and the wicked are two total opposites. When someone has escaped the path of wickedness he does not automatically qualify for the appellation of צדיק, a righteous person, until he has embraced the path of righteousness in his lifestyle. This is because the character of a wicked person is so totally different from that of a righteous person. When such a wicked person has abandoned his former path in life he has only reached the half-way mark separating him from the truly righteous person. This is why the rebuker has to be candid and explain to him the whole truth and not indulge in flattery, praising him for abandoning the path of evil, thereby lulling him into the belief that he has already completely rehabilitated himself. This is what prompted Solomon to say: "better open rebuke than concealed love.⁠" He may appear to reveal hatred by rebuking so candidly; however, better to appear in that guise than to appear as loving the subject of the rebuke by suppressing further guidance and congratulating the subject as if he already had completed the path of rehabilitation.
R' David Kimchi, (Radak) quoting his father, takes a different view of the meaning of our introductory verse. He understands Solomon as saying that an open rebuke is good; however it is so only if the rebuker reveals underlying love for the rebuked by speaking to him privately, not exposing him to embarrassment.
A Midrashic approach mentioned in Devarim Rabbah (Lieberman edition item 8) states that the "open" rebukes are the ones addressed by Moses to his people, whereas the ones described as "from hidden love,⁠" refer to the rebukes of Bileam who flattered Israel by pronouncing blessings, whereas in his heart he hated the very people upon whom he showered all his blessings. For instance, when he complimented the people on preserving each other's privacy by saying: "how goodly are your tents O Yaakov,⁠" etc., (Numbers 24,5) what he really wanted to say was: "how goodly are your tents O Moav, your dwellings, children of Ammon.⁠" Had God not twisted his mouth this is what would have come out of it.
There is yet another way to understand this saying by Solomon. The words: "open rebuke,⁠" refer to the rebukes we read in Leviticus chapter 26, whereas the ones in Deuteronomy, all of whom originate with the Lord Himself, though the wording may have been Moses', were approved by God as we explained already. The words מאהבה מסוטרת, may mean: "from the One whose love is not manifest, i.e. God, the attribute Shechinah.⁠"
This attribute is often referred to as אהבה, "love,⁠" such as in Proverbs 15,17: "better a meal of vegetables where there is love than a fattened ox where there is hate.⁠" The "love" referred to in that verse is a reference to the Presence of the Shechinah. The basis for such exegesis is found in Song of Songs 2,7: אם תעירו ואם תעוררו את האהבה, "do not dare to arouse the love,⁠" [a reference to premature attempts to secure the redemption, return to Israel of the Shechinah. Ed.] The word באהבה in Song of Songs 8,7 "that vast floods cannot quench love,⁠" also refers to the Shechinah. Similarly, the word אהבה, love, is used in our liturgy when we say להודות לך וליחדך באהבה, "to give thanks to You and to declare Your unity in the form of the Shechinah,⁠" (prior to reciting the Kriyat Shema). [The reason the word באהבה in that paragraph is not taken at face value, is presumably that the same word occurred immediately before the word להודות already, thus making it redundant unless it meant something different than usual. Ed.] Therefore, Solomon describes open rebuke as emanating from God's attribute Shechinah. Wherein then is the goodness of which Solomon speaks manifest? Even though the Israelites had violated so many of the commandments that God had been forced to bring upon them all the various afflictions mentioned both in Leviticus chapter 26 and in Deuteronomy chapter 28 so that the people were progressively deprived of the first Temple and the second Temple, all this was for their own good. Moses explained this to the people already in Deut. 8,21 when he said that God starved the Jewish people "in order to afflict you, to subject you to trials in order to treat you well in your distant future.⁠" All afflictions serve an educational purpose, to ensure that in the end you will become worthy of your lofty destiny. Although all the curses have been exhausted (fulfilled) i.e. Israel deserved the afflictions, the Jewish people have not ceased to exist. This is the meaning of Maleachi 3,7: "I the Lord have not changed, and you the Children of Yaakov have not perished.⁠" Our sages in Tanchuma Nitzavim 1 comment on this verse that when the Gentile nations experience a single series of afflictions this results in their demise from history, whereas the Jewish people, in spite of having experienced one holocaust after another, are still around and flourishing. They cite Micah 7,8 as proof of this. We read there: "Do not rejoice over me, O my enemy! though I have fallen, I rise again, though I sit in darkness the Lord is my light.⁠" Deut. 32,23 proclaims the same message when the Torah writes: "My arrows I have used up against them, yet they have not perished.⁠" The matter can be compared to a warrior who erects a beam shooting all his arrows at the beam. In the end, the warrior finds that he has exhausted his supply of arrows but that the beam is still standing. Although Israel has collapsed frequently under the afflictions God has disciplined it with, it always emerged from these afflictions and was rehabilitated. This is also the meaning of Jeremiah in Lamentations 3,12: "He has bent His bow and made me the target of His arrows.⁠"
This is also what Moses said in his admonitions here in the following verse:

אתם נצבים היום כלכם, ראשיכם, שבטיכם וזקניכם ושוטריכם כל איש ישראל, "You are all standing upright here his day, your chiefs, your tribes, all the men of your elders, your law-enforcers, all the men of Israel.⁠" All of the Israelites were assembled there in one place in anticipation of the renewal of the covenant just as they had all been assembled on the occasion of the first covenant at Mount Sinai. Seeing the people had been unfaithful to the first covenant when they had said (about the golden calf) "these are your gods O Israel" (Exodus 32,4), there had arisen the need to conclude a second covenant with this generation in the wilderness of Moav which the people had to confirm with an oath and a curse in the event that they would again dishonour their commitment. This is the reason Moses used the preamble: "you are standing here upright, all of you in the presence of the Lord your God.⁠" Moses meant to compare the present assembly to the one 40 years earlier at Mount Sinai. Whereas on that occasion Israel's word, unsupported by an explicit oath, had been deemed sufficient, this time they were required to confirm their undertaking by an oath.
It is possible to understand that our verse contains 2 promises: 1) that the Jewish people would endure and would never become extinct but would survive (as a people) all future catastrophes befalling mankind. 2) that the people would merit life in the hereafter if they would observe the commandments of the Torah. The need to give the people this assurance arose out of the fact that if they were to experience the full brunt of the curses mentioned in the previous portion, they would surely think that this spelled their doom. Moses therefore reminded them that just as they were all standing before him upright and well now, so they would emerge from even the worst aspects of the afflictions listed in the Torah and would be rehabilitated. The effect of the afflictions suffered in this life would be to erase the impact of their sins so that their claim to a life in the world to come would remain intact. This is what was meant when the Torah wrote in Deut. 6,25: "it will be a merit for us if we are careful to perform this entire commandment before Hashem.⁠" This is the assurance of life in the hereafter. This is also a meaning of the word כלכם which is an allusion to the statement of the sages in Sanhedrin 90 that כל ישראל יש להם חלק בעולם הבא, that "the entire Jewish nation has a share in the hereafter.⁠"

A Midrashic view (Tanchuma Nitzavim 1) of the words: "you are all standing here upright this day;⁠" just as this day commences in darkness, the darkness later giving way to light, so the darkness that you find yourself in as a result of enduring the afflictions listed previously will be replaced by bright light,⁠" as described by Isaiah 60,19: "the Lord will become for you an eternal light and your God will be your source of splendour.⁠"

ראשיכם שבטיכם, "your heads (leaders), your tribes, etc.⁠" The meaning is: "your heads, who are the leaders of your respective tribes.⁠" Moses listed the more distinguished people first, afterwards the adult males, followed by the male children who were under age, followed by the women, concluding with the proselytes.

כל איש ישראל, "every man in Israel.⁠" The sages in Tanchuma Nitzavim 1 use this expression to teach us that every man in Israel is co-responsible for the sin of every other Israelite. They bring practical proof for this from Joshua 7,11 where God told Joshua that the sin of one man, Achan ben Karmi, was the reason that the entire people had suffered a military setback at the hands of a small town, Ai. The words describing Achan's sin are: "Israel has sinned.⁠" This spells out the extent to which every Israelite shares in the sin of another Israelite. [Obviously, there are degrees in such sharing of another Israelite's sin. Ed.] This is also why we read in Leviticus 26,37: וכשלו איש באחיו, "they shall stumble one over the other.⁠" Our sages in Sanhedrin 27 interpret these words as meaning "they shall stumble over the sins of one another.⁠" This teaches the principle of reciprocal responsibility of one Israelite for the conduct of (all) other Israelites. If such reciprocal responsibility extends to all Israelites sharing misfortune on account of the sins committed by a few, how much more so will it apply to the reciprocal entitlement to enjoyment by all Israelites of good fortune (life in the hereafter), the entitlement to which was merited by the few! This is the meaning of the words of Eliphaz to Job ימלט אי נקי, "he will deliver the guilty; He will be delivered through the cleanness of your hands.⁠" (Job 22,30) In other words, a single individual with "clean" hands can be the instrument through which all the inhabitants of an island are saved. One single individual whose lifestyle is beyond reproach may save [for a period of time, Ed.] all his peers from impending doom. It follows that seeing while a righteous person is alive he saves his compatriots from doom, the death of such a man is a signal for all his compatriots to repent their sins, as they have been deprived of the protective merit of this righteous person. We have a principle that מדה טובה מרובה על מדת פורענות, that the range of the application of measures of good exceeds the range of the application of measures of evil. Hence if the righteous shares in the guilt of the sinner, the sinner benefits even more strongly from the merits accumulated by the righteous (Sotah 11).