Selecting between these words (relative pronouns, if you’re curious) is fairly straightforward as soon as you know how each one works. Here’s a quick rundown:
- Who: Now days, who is used exclusively for people and anthropomorphized (humanized) animals, fanciful creatures, or objects in fiction. Use this one even if the noun is simply a job title or a class of people (e.g., the soldiers who stormed the fort). However, in America, you don’t use who for an organization—even though it’s made up of people (e.g., the team that won the tournament, the acting group that came to our party).
- Which: Although there is some debate, which is most often used when a phrase following a non-human noun is not needed for understanding. That is, if you’re simply adding extra information about the noun, then use which and surround it with commas. How can you tell if it’s needed? Leave it out and see if the noun is still easily understood (e.g., Times Square, which is now closed to automobile traffic, is a place to work on your tan). The shades of gray can be broad here, but most are obvious.
- That: That is the same as which in being used for non-human nouns. The difference is how necessary it is. Use that when the phrase is needed for understanding (e.g., the dog that chewed up your paper, the book that she wrote). You never use commas with that phrases because they have to be there. Many times, you can also simply leave out that, since it’s implied (e.g., the book she wrote).
By the way, who phrases need commas only when they’re not needed for understanding. Here are two examples.
Mr. Rogers, who wanted to be your neighbor, sang on TV.
The pilot who flagged you down was not happy.