"Who" is a question word and the subject of the verb. Who is that? -> You're trying to find out the identity of someone. Who went to the store? "Whom" is similar to "who" but it can't be the subject of the sentence or is used after prepositions. You sat next to whom? You saw whom yesterday? This is the lady to whom I gave the gift. However, in spoken American English, very few people use whom. The majority of speakers use "who" in all contexts. So, if you never fully understand when to use "whom", it's ok. Most Americans don't either. "Whose" shows possession. You're trying to find out who owns something. Whose book is this? Whose house is that? "That" is a demonstrative pronoun that refers to something away from the speaker. That cat is pretty. (The cat is away from the speaker). "Which" is a question word asking for a choice between a group of objects or people. Which singer do you like the best? (I'm asking you to pick one from a group.) Which computer do you want? (Again, I'm asking you to pick one from a group.) "That" and "which" can also be used in the following ways: The books that have blue covers are the best. The books, which have blue covers, are the best. There really isn't a huge difference between the two. Using "that" is limiting which books are the best. Using "which" is adding information about which books are the best. However, most native speakers of American English use both words interchangeably. The difference is so small that it generally isn't a big deal. I hope I explained this well. If I didn't, please feel free to send me a message and I'll try to explain it better.