Of all the parts of speech, pronouns probably cause the most trouble. 

Do I use who or whom?

You can answer it if it is a question, or somehow try to substitute one of the nominative pronouns for who or whom.  If you would use I/he/she/we/they in the sentence, then who is correct.  Alternatively, if you would use me/him/her/us/them, you need to use whom.

Examples: 

1.     To (    ) did you give those tickets?  Answer the question:  I gave the tickets to him.  So you use whom.

2.     (      ) ate the pizza?  Answer the question:  He and I did.  So you use who.

·       Nominative pronouns – I, you, he, she, it, we, they

·       Objective pronouns – me, you, him, her, it, us, them

·       The pronouns “you” and “it” are the same for nominative and objective, so be careful.

TIP: Whom comes after by, with, for, to, between, from.