Exodus 22:20

And the stranger you shall not vex with words, nor distress him by taking his goods: Remember, sons of Israel, My people, that you were strangers in the land of Mizraim.

And a stranger – once the stranger accepts not to worship idolatry, you cannot oppress him in your country/land, because you are more powerful than him. And remember! You were strangers like him. And the same way that the text reminds you that the stranger does not have power, so too the widow and the orphans, who are Israelites, have no power. And after the text says you shall not wrong, it uses the plural form, but then it says if you do wrong them, uses the singular. This is because whoever sees a person oppressing an orphan or a widow and does not help the orphan and the widow, he too is considered as an oppressor. You must not mistreat any widow or orphan. Widows and orphans, like strangers, are in a disadvantaged position relative to other citizens.

AND A STRANGER SHALT THOU NOT WRONG, NEITHER SHALT THOU OPPRESS HIM; FOR YE WERE STRANGERS IN THE LAND OF EGYPT. There is no reason why all strangers [from countries outside the land of Egypt] should be included here because of our having been strangers in the land of Egypt! And there is no reason why they be assured for ever against being wronged or oppressed because we were once strangers there! Now Rashi explained that this is a reason for the prohibition against annoying a stranger. G-d warned against vexing him with words, for “if you vex him he can also vex you, by saying to you, “You also descend from strangers.’ Do not reproach your fellow man with a fault which is also in you.” Rabbi Abraham ibn Ezra explained the verses: “Remember that ye were strangers as he is now.” But there is in all these comments no real reason for the law.
The correct interpretation appears to me to be that He is saying: “Do not wrong a stranger or oppress him, thinking as you might that none can deliver him out of your hand; for you know that you were strangers in the land of Egypt and I saw the oppression wherewith the Egyptians oppressed you, and I avenged your cause on them, because I behold the tears of such who are oppressed and have no comforter, and on the side of their oppressors there is power, and I deliver each one from him that is too strong for him. Likewise you shall not afflict the widow and the fatherless child, for I will hear their cry, for all these people do not rely upon themselves but trust in Me.” And in another verse He added this reason: for ye know the soul of a stranger, seeing ye were strangers in the land of Egypt. That is to say, you know that every stranger feels depressed, and is always sighing and crying, and his eyes are always directed towards G-d, therefore He will have mercy upon him even as He showed mercy to you, just as it is written, and the children of Israel sighed by reason of the bondage, and they cried, and their cry came up unto G-d by reason of the bondage, meaning that He had mercy on them not because of their merits, but only an account of the bondage [and likewise He has mercy on all who are oppressed].

לא תונה, do not wrong him with words. In Leviticus 25,14 the Torah refers to similar legislation when applying to monetary matters. The same legislation applies to not wronging fellow Jews who are not converts, of course. The reason the Torah chose the convert as the example of the victim in our verse is that converts have much less of a chance to protect themselves against abuse of any kind, seeing they have no family to stand up for them if a wrong has been committed against them. Not only this, it is so easy to wrong a convert by mentioning what his parents do and what he himself used to do before he converted.

ולא תלחצנו, to perform work for you seeing he has no “redeemer” who looks out for his interest. G’d mentioned this to remind the Israelites who also had no one in Egypt to plead their case. (Exodus 3,9)

כי גרים הייתם, due to your personal experience of such a status, you, better than anyone else, know that seeing that the oppression of strangers is a great wrong, the punishment for violating such a commandment is equally harsh. (compare Exodus 23,9)