What was this stay on Har Sinai?
Rashi: It was to receive the second set of Luchos; and the Torah mentions it here to teach us how long he remained there. 1
Da'as Zekenim, Hadar Zekenim, Rosh: It was to receive the second Luchos and pray for a total pardon for Yisrael.
Rashi: Which it did not do above. (Verses 3 and 5 say only that Moshe ascended and descended.)
What are the implications of the words "ka'Yamim ha'Rishonim"?
Rashi: To teach us that these 40 days were days of goodwill, just as the first ones were. 1 However, Hashem was angry during the middle 40 days.
Ramban (citing the Seider Olam): To teach us that, during these 40 days, Moshe prayed to Hashem with the 13 Midos ('Hashem Hashem
Sifsei Chachamim: The first days, before Chet ha'Egel, were of total good will; the last days were not, for Hashem said "Salachti ki'Dvarecha" only at the end, on Yom Kipur. They are compared to the first days to teach that we do not discuss the middle days.
Ramban: And just as He did the first time, Hashem answered Moshe positively, as the next Pasuk explains.
What are the implications of "Lo Avah Hashem Hashchisecha"?
Ramban #1: Hashem did not want you to lose coming to and inheriting Eretz Yisrael. 1
Ramban #2 (According to Kabalah): It implies that you should not be banished from Hashem's great Name, that it not accompany you. 2