I’m used to being questioned about when to use “who” or “that,” but a question about “which” and “that” sent me scurrying to my grammar handbook. The two words have a couple of rules associated with them. The question this time, though, centered upon when to use one or the other.
To answer the question, “that” refers to most animals (usually unnamed animals) and inanimate objects.
The lion that escaped from the zoo was never found.
The fireworks that he bought were duds.
“Who” always refers to people and to animals with names.
Alice, the one who kills zombies and not the one who fell down the rabbit hole, is an interesting character.
Cheryl’s dog, Pepe, who likes to chase the neighbor’s cat, didn’t think it was too much fun to be chased by the other neighbor’s German Shepherd.
“Which” only refers to animals and things.
The river, which empties into the bay, has become polluted with toxic waste.
Her dogs, which run around the neighborhood at night, were caught by the dogcatcher.
The confusion with “which” and “that” is that they both refer to animals and things. The usual path to clarity is to understand when the word is essential or non-essential. “That” always introduces an essential clause; “which” typically is used with non-essential clauses. How to know when the clause is essential or non-essential? Try removing it from the sentence in question. If the sentence makes sense without the clause, the clause is non-essential and should be set off with commas. If the clause is essential, no commas should be used.
Do you have questions about “which,” “that,” or “who”? Let me know in the comments. Also, if you have a grammar question, let me know on my Facebook page.