Probably one of the most versatile words in the English language is that. That can be used as a pronoun (That is my sister), as an adjective (You can sit in this chair or that one), or as an adverb (The movie was that good). That can express a time or an event (After that we went to the movies), describe a remarkable person (She’s all that and a bag of chips), or make something sound exciting (Now, that’s more like it). We use that every day for "this and that," and when we’re done, we say, "That’s that."
Sometimes, however, people wonder if they should be using which instead of that.
Which can also be used as a pronoun and as an adjective just like that; however, people often confuse the two when they need one to act as a relative pronoun when introducing a clause.
Sound confusing? Well, I can make it way simpler! First, let’s start with an example of that and which used correctly in a sentence.
The house that has the Adarondack chair out front belongs to my sister.
East End Pool, which is near my house, stays open later into the season than Smalley Pool does.
You should use that when the information provided is critical to the understanding of the message.
The dog that has the pink collar was adopted from a rescue. The information about the pink collar is critical to understanding the message, so that is correct.
You should use which when the information provided is not critical to the understanding of the sentence.
The cake Laurie made, which was the biggest I’ve ever seen, tasted great! The size of the cake has no bearing on how tasty Laurie’s cake was, so which is correct. For these types of clauses, you should always put a comma before the word which—besides being grammatically correct, it is your indicator that what you are about to describe is nonessential to the meaning of the message.
When I was getting my master's degree at Elmhurst College, I remember my professor telling the class that if a person is a native English-speaking American, she will always naturally make the right choice when it comes to using that and which.
This information stuck with me, because I get several e-mails every month from co-workers asking me, "Should I use that or which in this sentence?" So, I started to think that maybe there is an exception to that tidbit my professor shared with us. Maybe when people speak, they naturally make the correct choice, but when writing, people take the time to question themselves and worry that they’ll get it wrong.
Since then, however, I have learned that the purists (meaning the British) do not recognize this rule at all—the British use that and which in this situation interchangeably. In fact, there are many grammar rules on which the British and Americans do not agree. So perhaps my teacher was really trying to tell us there is no wrong answer.
My advice? Since we live in the United States, we should default to American grammar. If you are unsure about which word to use, say your sentence out loud, then write it down. Considering all of the e-mails I have received over the years regarding the use of that and which, I have never come across one that was wrong.