Who v Which v That
Rule 1
Who refers to people. That and which refer to groups or things.
Examples
Anya is the one who rescued the bird
Lokua is on the team that won first place
She belongs to an organization that specializes in saving endangered species.
Rule 2
That introduces essential clauses while which introduces nonessential clauses.
Examples
I do not trust editorials that claim racial differences in intelligence.
We would not know which editorials were being discussed without the that clause.
The editorial claiming racial differences in intelligence, which appeared in the Sunday newspaper, upset me.
The editorial is already identified. Therefore, which begins a nonessential clause.
Note
Essential clauses do not have commas surrounding them while nonessential clauses do contain commas.
Rule 3
If this, that, these, or those has already introduced an essential clause, use which to introduce the next clause, whether it is essential or nonessential.
Examples
That is a decision which you must live with for the rest of your life.
Those ideas, which we've discussed thoroughly enough, do not need to be addressed again.
Note
Often, you can streamline your sentence by leaving out which.
Example Those ideas, which we have discussed thoroughly, do not need to be addressed again.
Better The ideas we have discussed thoroughly do not need to be addressed again.
Example That is a decision which you must live with for the rest of your life.
Better
That is a decision you must live with for the rest of your life.
or
You must live with that decision for the rest of your life.
