A.
The word which can be used to introduce both restrictive and nonrestrictive clauses, although many writers use it exclusively to introduce nonrestrictive clauses; the word that can be used to introduce only restrictive clauses. Think of the difference between
- "The garage that my uncle built is falling down."
and
- "The garage, which my uncle built, is falling down."
I can say the first sentence anywhere and the listener will know exactly which garage I'm talking about — the one my uncle built. The second sentence, however, I would have to utter, say, in my back yard, while I'm pointing to the dilapidated garage. In other words, the "that clause" has introduced information that you need or you wouldn't know what garage I'm talking about (so you don't need/can't have commas); the "which clause" has introduced nonessential, "added" information (so you do need the commas).
Incidentally, some writers insist that the word that cannot be used to refer to people, but in situations where the people are not specifically named, it is acceptable.
The students that study most usually do the best.
(But we would write "The Darling children, who have enrolled in the Lab School, are doing well.")
B.
Rule 1.
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Who refers to people. That and which refer to groups or things.
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Examples:
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Anya is the one who rescued the bird.
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Lokua is on the team that won first place.
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She belongs to an organization that specializes in saving endangered species.
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Rule 2.
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That introduces essential clauses while which introduces nonessential clauses.
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Examples:
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I do not trust products that claim "all natural ingredients" because this phrase can mean almost anything.
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We would not know which products were being discussed without the that clause.
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The product claiming "all natural ingredients," which appeared in the Sunday newspaper, is on sale.
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The product is already identified. Therefore, which begins a nonessential clause.
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NOTE:
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Essential clauses do not have commas surrounding them while nonessential clauses are surrounded by commas.
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Rule 3.
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If this , that , these , or those has already introduced an essential clause, you may use which to introduce the next clause, whether it is essential or nonessential.
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Examples:
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That is a decision which you must live with for the rest of your life.
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Those ideas, which we've discussed thoroughly enough, do not need to be addressed again.
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NOTE:
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Often, you can streamline your sentence by leaving out which .
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Example:
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That is a decision which you must live with for the rest of your life.
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Better:
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That is a decision you must live with for the rest of your life.
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