My second question is: In simple terms, when do you use who and when , do you use whom? How do you differentiate? Thank you.
Who/whom are treated very much like other pronouns. "Who" is in the nominative (subjective) case and "whom" is in the objective case. One would use "who" when it is the subject of a sentence or clause. One would use "whom" when it is the object of a verb or a preposition. This is the same as he/him, she/her, and I/me.
The only tricky parts with who/whom are:
1. It is standard English to use "whom" as the object of a preposition, even when the pronoun precedes the preposition. In spoken or informal English, this does not always happen.
Standard: Whom do you wish to speak to?
Standard: To whom do you wish to speak?
Informal: Who do you wish to speak to?
2. Use "who" as the subject of a clause even if the clause is the object of a verb or a preposition.
"It was only then that I learned who had sent the letter."
Even though the clause is the object of "learned", "who" is the subject of that clause.
3. Use "whom" to introduce a clause is "whom" is the object.
"It was only then that I told the group whom I suspected."