Challenges with the English Language.

The areas of the English Language that I tend to have trouble with are deciding whether to use a comma or a semicolon, the use of who and whom and the difference between that and which. I understand semicolons should only be used when you are trying to separate two complete ideas in a sentence. I also understand that the word who is supposed to be the subject of a sentence while whom is the object. Unfortunately, compound and sentences that are worded in a strange manner often make it difficult to choose which way to go with either of these questions. As for the question of using that or which, I still have no idea when to use either of them. I guess it's time to open up my Stylebook.