YSK: If you struggle with when to use who/whom; If you can replace the word with “he”' or “'she,” use who. If you can replace it with “him” or “her,” use whom.
from /r/YouShouldKnow
YSK：如果你为何时使用who/whom而苦恼；如果你能用 "他"'或"'她 "来代替这个词，就用who。如果你能用 "他 "或 "她 "代替，就用who。