Ah, the endless confusion over whether to use “that” or “which.” Hopefully I can break it down for you so you’ll never be confused again!
The first thing to remember is that the word which is almost always preceded by a comma, parenthesis, or a dash.
The word that is not preceded by a comma, parenthesis, or dash (usually).
-Which is used non restrictively—not to narrow a class or identify a particular item but to add something about an item already identified. (By the way, did you notice the not/but rule coming to life from last week’s grammar tip?)
-That is used restrictively to narrow a category or identify a particular item being talked about.
Example: Any building that is taller must be outside the state / Alongside the officer trotted a poodle, which is hardly a typical police dog.
In the case of the first sentence, “that” is identifying a particular item–ie, a taller building. In the second sentence, the word “which” signifies the addition of information. The poodle has already been identified, and the “which” phrase is simply providing additional information.
Here’s another way to think about it:
In the above example, you can remove the phrase “which is hardly a typical police dog,” and you still have a sentence that makes sense: Alongside the officer trotted a poodle.
However, in the first example, there is no way to remove any part of that sentence and still have it make sense. You can’t say: Any building must be outside the state. The “that is taller” clause is necessary to keep the same meaning. Using “which” is a similar idea to using parenthesis. When used correctly, you should be able to remove the parenthesis and still have a complete sentence. The same is true for the “which” clause, in the majority of cases.
Another example: My bike that/which I keep in the garage.
Which is it?
Based on what I’ve just explained, “that” is correct. But consider this: My bike, which I keep in the garage, is broken.
In this case, you would use “which” with a comma, because, as stated earlier, you could remove the phrase between the commas and have a perfectly good sentence: My bike is broken. The “which I keep in the garage” is just additional information.
Most of the time, “which” is going to be serving the purpose of providing that bit of extra information and will need a preceding comma. But there are times when “which” does not follow a comma: when it is preceded by a preposition.
Example: She wrote him a letter in which she stated that she did not love him anymore.
All that to say, there seems to be a bit of argument on both sides for the general “correctness” or need for a distinction between “that” and “which.” This article seems to make the argument that using that vs. which is merely a difference of American vs. British English. However, Grammar Girl agrees with me.
What do you think? Is this something you notice when you’re reading?