Something I often see while editing is confusion about when to use which, that, and who or whether to have a comma before it or not. Like most things in English, there are nuances that muddy the water, but here’s some simple tips.
Use who to refer to people (rather than objects) whenever possible. For example:
I like people who like animals.
You can use a comma before who, but this depends on what you’re conveying (see below).
Use that without a comma when talking about objects and want to limit the scope of the noun you’re referring to. For example:
I like cars that are fast.
In this example of a restrictive clause, I’m only saying that I like fast cars. Maybe I like slow ones too, or maybe I don’t. One simple thing to look out for when using that is that it should rarely have a comma before it, because it can only be used to start a restrictive clause (see below).
In the strictest interpretations,which should only be used to begin nonrestrictive clauses. These are clauses that provide additional information about the noun rather than restricting their scope. For example, if I said,
I like Ferraris, which are fast
I would be saying that I like all Ferraris. I would also be saying that all Ferraris are fast. You would typically use this when your noun is already specific enough. Note that when you use which like this, it should always have a comma before it.
A Comma Before?
Using a comma before generally indicates the start of a nonrestrictive clause. Omitting it indicates the start of a restrictive clause. This is especially important when using who because it can subtly change the meaning. For example, use a comma when you just want to convey extra information such as:
I’m a fan of Sting, who used to be in the Police.
Like most things, there are exceptions. You’ll see many best-selling novelists use which instead of that, and British English especially does not prohibit this. The difference is there won’t be a comma before, which indicates a restrictive use of which. To be safe, I generally stick to the rule above for clarity.
Another exception is that grammar rules often allow for people to be referred to using that rather than who. Technically, this is allowable, but many people cringe at the sight of it and most style guides forbid or discourage it, so only use it as a last resort. I should probably also point out that which should never refer to people.