Rule 1. Who refers to people. That and which refer to groups or things.

Examples Glen is the one who rescued the bird.
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 like editorials that argue for racial differences in intelligence.
We would not know which editorials were being discussed without the that clause.

The editorial arguing for 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 that has already been used in the sentence, use which to introduce the essential clause that follows.

Example That is a decision which you must live with for the rest of your life.

Rule 4. If the essential clause starts with this, that, these, or those, use which to connect.

Example Those ideas which were discussed on Tuesday will be put in the minutes of the meeting.

Even better The ideas discussed on Tuesday will be . . .

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