In my writing classes, students often want to know the rules about when to use who, that, and which in a sentence. I tell them that confusion about using these is not uncommon, and I begin my explanation with a discussion of the differences between who and that/which.
Use who when referring to a person. Consider this example: The boy who is at the end of the line should come to the front now. Use that or which only when referring to places or things. Take these examples: The sandwich that he ate was delicious. or The sandwich, which I ate yesterday, was delicious. Obviously, we would never say the sandwich who; nor should we say the boy that (although using that when referring to a person—particularly in conversation—is one of the more common grammatical mistakes).
Once they understand how to use who, that, and which correctly, many people still are confused about whether they should use that or which when referring to places or things. To understand which one is the correct choice, though, you first need to know the difference between a restrictive clause and a nonrestrictive clause.
A restrictive clause is a part of the sentence (a clause) that is essential to its meaning. For instance, in the previous example (The sandwich that he ate was delicious.), the clause that he ate is needed because I want my reader to know that we are not just talking about any sandwich; we are talking about the one that the boy ate. So, we use the word “that” to begin a clause that is essential if we are to understand the meaning of the sentence.
A nonrestrictive clause is one that adds more information to a sentence, but this information does not need to be there for us to understand the sentence’s meaning. And, to introduce an inessential, nonrestrictive clause, we use the word “which.” Consider, again, the example above: The sandwich, which I ate yesterday, was delicious. As the author of this sentence, I want to use the word “which” because the clause which I ate yesterday is additional information that is not critical to understanding the sentence, and I can take it out without changing the meaning.
We would use the word “who” to begin a nonrestrictive clause when we are referring to a person (e.g., The boy, who walked home yesterday, was happy.) Who walked home yesterday, the nonrestrictive clause, can be removed from the sentence, and it still would make sense.
Sometimes, though, our use of that or which depends on the meaning we intend. For instance, if I write The sandwich that I ate yesterday was delicious., I am using that because it is essential to the meaning of the sentence. If, say, previously I have mentioned something about a sandwich I ate today, which was not delicious, and I want to compare it to the one I ate yesterday, then that I ate yesterday is a necessary (restrictive) clause if you are to understand the comparison I am making.
Or, if I write Yesterday I rode the bus that is blue, by using that and not which I am suggesting there are other buses that are not blue. So, my next sentence could be Today, I rode the bus that is pink.
One last point is that we need to place commas at the beginning and at the end of a nonrestrictive clause, but we never use commas around a restrictive clause. Here are a few examples:
- The girl, who is wearing green, came to school late.
- Our car, which is black, is very dirty after the snowstorm.
- The glasses, which I bought at Target, will be perfect for the holidays.
- BUT NOT The sandwich, that I ate, was delicious.
So now you know.