What is the connection between this Pasuk and the previous one?
Seforno: The nicest way of supporting a needy person is by extending him a free-interest loan.
What is the difference between "Neshech" and "Tarbis"?
Rashi (from Bava Metzi'a 60b): In fact, the Chachamim equate them, and the Torah gives them two names to make them two Lavim.
Ramban, Moshav Zekenim: The simple meaning would be that "Neshech" 1 is interest that increases every year, and Tarbis is lending on condition to pay back with an addition, which does not depend on when he repays. 2
Ramban: So-called because the interest grows into a large amount - like the blown-up swelling of a snake-bite. This is customary with a money loan. See Ba'al ha'Turim.
Ramban: As one tends to do with fruit, where Reuven lends Shimon a measure of food until the harvest, when Shimon repays a little extra.
Why does the Torah insert the words "v'Yareisa me'Elokecha"?
Rashi: Because a person is drawn after interest, and it is difficult to break away from it. Moreover, one allows oneself the liberty of taking interest to offset the loss of his money. 1
Rashi (from Bava Metzi'a 61b): It is warning against lending money on interest claiming that it is a Nochri's. 2 Only Hashem knows the true facts. Therefore the Torah writes "v'Yareisa me'Elokecha".
What are the ramifications of the phrase "v'Chei Achicha Imach"?
Ramban #1: "Imach" implies that your life takes precedence over that of your fellow-Jew. This teaches us the Mitzvah of self-preservation. 1
Ramban #2 (citing Bava Metzi'a, 62a): It implies an obligation to return a fixed Ribis to the borrower, to enable him to live.
Seforno: It implies that one is only obligated to lend a needy person money if, after extending the loan, one has enough to live on oneself. 2
Rashi (in Kesuvos 15b) and Targum Onkelos: It is a Mitzvah to sustain one's fellow-Jew.
It is a reason for the prohibition against taking interest - because he is your brother and you would not lend your brother money on interest. (EC)
Ramban (citing the Toras Kohanim): If Reuven and Shimon were traveling in the desert, and Reuven had sufficient water for one person, Ben Putiri ruled that they should share it based on "v'Chei Achicha Imach." R. Akiva countered that "Imach" teaches that your own life comes first. The repetition comes to emphasize the issue. (I.e. we could have learned from the previous verse; "va'Chai Imach" applies also to Achicha mentioned earlier - PF.) See also R. Chavel's footnotes.
Seforno: As the Gemara states in Bava Metzi'a, 62a 'Your life takes precedence over somebody else's.
QUESTIONS ON RASHI
Rashi writes that one may not lend money on interest claiming that it is a Nochri's. Even if it is a Nochri's it is forbidden, for Shelichus does not apply to Nochrim (it is as if the Jew lends)!
Riva: If Reuven tells Shimon 'take this security and borrow from a Nochri with Ribis', it is permitted, for Shimon has no liability to the Nochri; he is Reuven's Shali'ach. Or, if a Nochri deposited coins with Shimon, and he has no responsibility for them, the Yerushalmi says that he may lend them with Ribis. This fits better into Rashi's words.