Who/that - for persons which/that - for things and objects 1. The computer is the machine which/that is necessary today. 2. My brother, who lives in LA, is a businessman. 3. Do you know whose is that dog? 4. Whom you met yesterday? I hope I helped you.

The most difficult of these questions, for native and non-native speakers, is who/whom. The rule is that who is subject, whom is object. The test is: if you can use him in the sentence, then use whom. Here are some examples:http://www.grammarbook.com/grammar/whoVwhom.asp

which,who,whose,whom are called as interrogative words used for questioning. WHICH - used in questions etc when asking someone to point out, state etc one or more persons, things etc from a particular known group. Which (colour) do you like best?; Which route will you travel by?; At which station should I change trains?; Which of the two girls do you like better?; WHO - what person(s)(?). Who is that woman in the green hat?; Who did that?; Who won?; Do you know who all these people are? WHOSE - belonging to which person(?). Whose is this jacket?; Whose (jacket) is this?; Whose car did you come back in?; In whose house did this incident happen?; Tell me whose (pens) these are. WHOM - the objective case of who, used as a direct or indirect object: Whom did you call? You gave whom the book?