Why does the Torah insert the word "Im Kesef Talveh ... "?
Targum Onkelos and Targum Yonasan: Refer to 21:30:1:2. Similarly here, the Torah is stating that in the event that one lends a poor man money, one may not behave like a creditor or lend him on interest. 2
Seforno: It is only in a situation of "Ki Lo Yechdal Evyon mi'Kerev ha'Aretz" (Devarim 15:11), but not in one of "Efes Ki Lo Yih'yeh b'cha Evyon" (Ibid., 15:4).
Rosh citing R. Yehudah of Paris, Da'as Zekenim, Hadar Zekenim and Riva, all citing R. Yehudah ha'Chasid: Sometimes there is no obligation to lend, e.g. to one who borrows and does not pay. Riva - the other two Chiyuvim about which it says "Im" (refer to 20:22:1:1), this is because the Chiyuv is only while the Mikdash stands.
Oznayim la'Torah: Because Parshas Mishpatim deals with money that can be claimed in Beis-Din, whereas Tzedakah belongs to Parshas Re'ei, which deals with similar matters. And the Torah only mentions it her to teach us that, if one does lend money, one should do so freee of interest.
Why does the Torah insert the words "Ami" and Imach"? Why does it not simply write "Im Kesef Talveh Ani"?
Rashi #1: To teach us the order of priorities of giving loans - 1. To a fellow-Jew; 2. To one who is poor; 3. To one who is with you (in the same town). 1
Rashi #2: The Torah writes "Ami" to indicate that when giving a loan to a fellow-Jew, one should bear in mind that he is one of yours and not give contemptuously, and "Imach" to teach us that when lending to a poor man, one should consider oneself to be poor.
Hadar Zekenim, from Tanchuma: We read "Es Ami" like "At Imi" - one who lends an Oni merits to enter the boundaries of Hashem's abode.
Divrei Eliyahu: A loan should be in front of witnesses (lest the borrower deny it), so it says "Es Ami." Tzedakah should not be in front of others (lest the Oni be ashamed), so it says "Es he'Ani Imach".
Oznayim la'Torah: "Ami" refers to a total Ani, 2 who has nothing, neither where he lives nor in any other location, neither now, nor later, in the form of a debt that is owed to him or from other source; whereas ""he'Ani Imach" refer to someone who is currently poor, but who either has money in another location or who has money coming to him at a later date. The Torah here is giving precedence to the former, who will finf it much more difficult to obtain a loan.
What is the meaning of "Lo Sih'yeh lo ke'Nosheh"?
Rashi #1: Don't demand payment if you know that he doesn't have the funds to pay. In fact, one should in no way (embarrass him by) behaving towards him as if he owes one money. 1
Rashi (in Bava Netzi'za, 75b): Do not appear to him like a creditor
Why does the Torah call interest 'Ribis'?
Rashi: Because, like a snake-bite, which is initially small, but swells suddenly, so too interest, which begins with a small sum, but which, before one realizes, turns into a large amount. 1
Why does the Torah forbid taking interest?
Following the La'av of Ribis, the Pasuk writes in B'har, Vayikra, 25:36 "V'chai Achicha Imach", indicating that one does not take interest from a fellow-Jew because he is considered a brother
As opposed to borrowing vessels (which one returns intact, and) which are (therefore) not included in the prohibition, 'Milveh' ("Im Kesef Talveh') implies a loan of money, which is meant to be spent and replaced (Rashi, Avodah-Zarah, 2a).
And not one of personal benefit, either monetarily or for Kavod (Ramban).
Why does the Torah use the expression "Lo Sesimun... "?
Targum Yonasan: To incorporate the witnesses and the guarantor in the La'av. 1
Bava Metzi'a, 75b: To incorporate in the La'av the borrower, the guarantor, the witnesses and the Sofer.
Oznayim la'Torah (citing Bava Metzi'a, 75): To render him as having transgressed the La'av already from the time that he obligates the debtor to pay (when he writes the Sh'tar)..
See also Ba'al ha'Turim.
Why does it say "Lo Sesimun Alav++ Neshech"?
Moshav Zekenim: The Torah forbids only Ribis paid from the borrower to the lender. It is permitted as long as the lender gets no benefit from him. 1
QUESTIONS ON RASHI
Rashi writes that it says "Ami" to teach that one must lend to a fellow-Jew before to a Nochri [with Ribis]. We should know this already from Neveilah. The Torah commands to give to a Ger [Toshav] before selling to a Nochri, and all the more so here!
Riva: Had the Torah taught only about Neveilah, which is worth little, we would not know a loan, which is a great profit (the Ribis). Had the Torah taught only about loans, in which he does not lose any [principal], we would not know Neveilah (that he should give it for free to a Ger Toshav). 1
Also, we cannot learn from here that one should accept a loss in order to benefit a Ger Toshav! (PF)