“Whom” or “who” in this context? [duplicate]

Question

This question already has an answer here:

• What’s the rule for using “who” and “whom” correctly? 11 answers

I'm not a native speaker so my apologies if this question is dumb. Which one of these is correct?

• I didn't know what to do, so I asked my sister whom has done this before
• I didn't know what to do, so I asked my sister who has done this before

I guess, the first one is correct.

Answer 1

Because "sister" is the subject of the phrase "...has done this before", the correct pronoun is "who", since "whom" is used for the indirect object of a sentence.

You could test which phrase you're after by trying to substitute "she" for "who" or "her" for "whom". This would yield the potential results "SHE has done this before" (which is correct!) or "HER has done this before" (which is obviously wrong, and sounds like a funny caveman's way of speaking).

And you need to insert a comma before the phrase "who has done this before", so you get "...my sister, who has done this before.".