That and which – you see and use them all the time. My college grammar professor once explained that we naturally speak these words correctly, but when writing, they are misused. I think because we are raised to use them correctly we never stop to think about the rules. So, for those who need a refresher, here’s a quick guide on how and when to use that and which.
Common terms used in this post:
Clause: a sentence or a sentence-like section within a sentence
Essential clause: A clause that needs to be there in order for the sentence to make sense. Without the clause, the sentence’s meaning would be unclear. (It is essential to the sentence).
Nonessential clause: A clause that is not important to the meaning of the sentence. It is often additive. (It is nonessential to the sentence’s meaning).
Rule 1: Who refers to people. That and which refer to groups and things. For animals, this rule depends on the style guide you are using and varies. I prefer who for domesticated animals.
Dr. Quain is the one who taught me these rules.
Dr. Quain belongs to the faculty that assisted me with graduation.
It was the cat who hurt his foot.
Rule 2: That introduces essential clauses. If you take everything after the word that/which out, and the sentence loses its meaning or doesn’t make sense, use that.
I’m going to buy the car that has the best gas mileage.
He only watches shows that make him laugh.
Rule 3: Which introduces nonessential clauses (place comma before when using). It is also used if a selection or decision is implied (comma not needed).
We watched him go down the hall alone, which darkened the further he went.
We won the game, which puts us in first place.
I don’t know which road to take.
Rule 4: If this, that, these, and those have introduced an essential clause, use which.
These designs, which do not have approval, will be tough to apply.
That dog doesn’t know which bone to eat.
The Blue Book of Grammar and Punctuation, Grammar Girl, my brain
If you have any other tips, feel free to share them below!