לא תתחתן בם, “do not intermarry with them.” The nations referred to are the ones mentioned in Kings I 11,1.

כי יסיר, “for he will turn away;” this is the son of a Canaanite marrying a Jewish girl, mentioned in verse three. The father of such a child will bring him up as a heathen, although legally, he is a Jew, his mother being Jewish. In the case of the child being a daughter, this will be even more so, as living as a heathen is so much easier for the children who do not have to observe all the commandments of the Jewish religion. (B’chor shor)

לא מרובכם מכל העמים, “not because you were more numerous than all the other nations;” the “other” nations referred to here are the seven Canaanite tribes that will be replaced by the Israelites shortly.

כי אתם המעט מכולם, “rather because you are the least numerous;” but it is because He loves you.

וידעת כי ה' אלוקין־, “for you know that He is the Lord your G-d;” this is a continuation of the two previous verses. The reason why you are so certain that Hashem is the Lord your G-d is because you are aware of what He has done for your ancestors commencing with Avraham. He has faithfully kept all the promises He has given them. (B’chor shor)

לאלף דור, “for up to a thousand generations.” Why did these words have to be added? Having told you that G-d loves you, you might think that you do not have to bother to keep all the commandments of His Torah, seeing that G-d is obligated to keep His oath to our forefathers, regardless. G-d is willing to wait with keeping His promises for as long as a thousand generations, if in the interval no generation had proved worthy of what He had promised to the forefathers.

שומר הברית, “He keeps His covenant;” the word שומר here means: “waiting in anticipation,” as it did in Genesis 37,11 where Yaakov was described as awaiting fulfillment of Joseph’s dream with the words: ואביו שמר את הדבר, “and his father remained in expectation of what would happen.”

ומשלם לשונאיו אל פניו, “and He repays them that hate Him to his face.” He punishes such people for their sins during their lifetime.

אל פניו ישלם לו, “He pays him in his lifetime,” instead of waiting to punish his children if they continue in their father’s way. If they do not do so, (continue to sin) they will not be burdened with the sins of their fathers. This is spelled out clearly in Ezekiel 18,17: והוא לא ימות בעדן אביו, “but he will not die on account of the sin of his father.”

ושמרתם את המצוה, “You shall therefore keep the commandment;” your observing His commandments will be appropriate as a repayment for G-d keeping His promises to the forefathers.”

והיה עקב משמעון, “it shall come to pass as a result of your hearkening, etc;” Moses tells the people that on account of their observing the laws of the Torah the blessings following will be bestowed upon them collectively. On the other hand, if you do not observe His commandments He will not fulfill the terms of His covenant until the following generation.

הצרעה, “the hornet;” according to some interpretations this is a type of disease similar to tzoraat, afflicting the body. (Ibn Ezra)

לא תוכל כלותם מהר, “you will not be able to destroy them quickly;” the word תוכל is not to be understood as an inability on your part. It will not be in your interest to do so, as you would create a vacuum of not enough people in the land, that would encourage invasion by free roaming beasts.

תשרפון באש, “you are to burn them by fire.” When the Israelites, after seeing the golden calf, addressed it with the words: “these are your deities (pl) Israel!” all types of idolatries were included in that declaration, and there is a rule that the only way an idolatry practiced by a Jew can be nullified is by destroying the idol by burning it. (Compare Talmud tractate Menachot folio 53) The only other way is to reduce it to dust and throw the dust to the winds.

פן מוקש בו, “lest you be ensnared by it.” Sometimes one hesitates to destroy such images completely as one does not wish to destroy the value of the gold or silver that was part of it. Moses warns that by harbouring any part of it you are inviting disaster to befall your home and family.

שקץ תשקצנו, “you are to treat it as if it were an abomination (regardless of its market value). The reference is to residual gold and silver of such deities even after they have been dismantled.