Having already taught the Mitzvos of Pe'ah and Leket (see 19:9), why does the Torah repeat them here?
Rashi: To sentence the transgressor to two sets of lashes:
Refer to 23:22:2:1-4.
Seforno: After thanking Hashem for a successful harvest, the Torah issues a warning on how to succeed in preserving one's gains - by giving Leket and Pe'ah. 1
Seforno: As the Gemara states in Kesuvos, 66b 'Melach Mamon Chaser' or 'Melach Mamon Chesed'.
Why were the Mitzvos of Pe'ah and Leket put in the Parshah of the Regalim?
Rashi: To teach us that when someone leaves Leket, Shichechah and Pe'ah, it is as if he built the Beis-Hamikdash and brought Korbanos in it.
Ramban, Moshav Zekenim (both citing Ibn Ezra): Bearing in mind that Shavu'os begins the wheat harvest season, the Torah is issuing a warning not to forget to carry out the Mitzvos that pertain to that time of year.
Ramban: With reference to the beginning of the Parshah, 1 the Torah is teaching that, even when cutting the barley for the Omer, the Mitzvah of the Korban Omer does not override the Lavim of Leket and Pe'ah, and that one is nevertheless obligated to leave them for Aniyim.
Rashbam: Because it comes into effect after bringing the Omer, when one begins to cut the harvest. 2
What are the implications of the words "Ta'azov osam?
Rashi: It implies that one should leave Pe'ah in the corner of the field for the poor to cut, and not to assist them to collect it. 1
See Sifsei Chachamim.
Why does the Torah omit Shichechah?
Refer to 19:9:4:1 and the note there.
Why does the Pasuk conclude "Ani Hashem Elokeichem"?
Rashi: To teach us that Hashem will reward the person who leaves Pe'ah and Leket in his field.
Rashi (19:2): Refer to 19:2:2:1*.
Seforno: Hashem is the G-d of both the harvesters and of the poor who collect the Pe'ah and the Leket; and He will reward those who do good to the latter in fulfillment of His wishes.