I do a lot of editing for other writers. One of the most common mistakes I see in manuscripts is the use of the word which in place of the word that in sentences.
Look at this sentence:
I went to the store which sells CDs at half price.
It should read:
I went to the store that sells CDs at half price.
If we remove the information starting with which or that, we're left with I went to the store. All we know now is that I went to a store. What we don't know is, what kind of store? A food store? A drug store? No! It's the one that sells CDs at half price.
Sells CDs at half price is vital information that's needed to understand the total meaning of the sentence. Information that is vital to a sentence -- that can't be left out without changing the meaning of the sentence -- is called a "restrictive element". It "restricts", or "limits" the sentence to one specific meaning.
The word that is used to indicate a restrictive element.
A "non-restrictive element" is non-vital information that can be left off without changing the meaning of the sentence. The word which is used to indicate a non-restrictive element.
Here's how we could change the above sentence to show a non-restrictive element.
I went to the store, which sells CDs at half price, to buy the latest Shania Twain CD.
The phrase which sells CDs at half price is not vital to the sentence. It doesn't change the meaning of the sentence if left out, so we call it a non-restrictive element. In short, we can understand that I went to the store to buy the latest Shania Twain CD without being told that the store sells CDs at half price.
In short, the word that is used with restrictive -- or vital -- elements. A comma does not precede that when used with restrictive elements. The word which is used with non-restrictive -- or non-vital -- elements. A comma does precede -- and often follows -- the word which when used with non-restrictive elements.
Hope this editing tip helps you. Good luck with your "whiches" and "thats"! :)