What is "Lo Soneh"?
Rashi: It is tormenting verbally.
Why does the Torah mention specifically a Ger?
Rashi: Because if you taunt him, he can point a finger at you and remind you that you too, were strangers in Egypt. 1
Ramban #1 (citing Ibn Ezra) and Rashbam: Remember that you too, were strangers in Egypt (and how much you suffered as a result)! 2
Ramban #2: Refer to 22:20:3:2.
Rashbam: Because, having no close relatives to take his part, he is vulnerable. 3
Rashi (23:9, from Bava Metzi'a 59b): It is because Suro Ra, (I.e. his source is bad. Perhaps he will return to his bad ways.)
As the Pasuk goes on to state.
The Ibn Ezra seems to interpret the Pasuk with regard to a Ger Toshav. If that is how Targum Yonasan interprets it, it will explain why he writes that he is Chayav Hereg (the Misah that Nochrim receive) and not Sekilah (the Misah that is designated for Jews who worship Avodah-Zarah (See Na'ar Yonasan).
What is the connection between not oppressing a Ger and the fact that Yisrael were strangers in Egypt?
Ramban #2: The Torah is issuing a warning here against thinking that, because a Ger has nobody to support him, he is helpless and therefore one may take advantage of him and oppress him at will. Not at all! One should remember how we were strangers in Egypt and how Hashem rescued us when we cried out to Him for help, 2 because it is His way to always help the underdog. 3 Consequently, if we oppress the Ger and he cries out to Hashem, He will defend him too.
One does not remark on the blemish in somebody else if one possesses the same blemish (Rashi).
And the same reasoning extends to the widow and the orphan, as the Torah indicates in Pasuk 22 (Ramban). In fact, the Seforno, connecting this Pasuk with thr following Pasuk, presents the same explanation as the Ramban with regard to Yasom and Almanah.
What is "v'Lo Silchatzenu"?
Rashi: Do not steal from him.
Rashbam: Do not coerce him to do your work.
Targum Yonason: Do not force him to hand his property over to you.
Why is this written right after idolatry?
Hadar Zekenim (20): If a Nochri served idolatry and converted, do not verbally pain him for what he did, for everything was pardoned.