Today's English lesson

From EnglishPlus:

That, Which, and Who

That, which, and who when used as relative pronouns each have a distinct function.

In modern speech, which refers only to things. Who (or its forms whom and whose) refers only to people. That normally refers to things but it may refer to a class or type of person.

Examples:

That is a book which I need for the class.

These are the books that I need for the class.

He is the man who will be teaching the class.

They are the type of people who would lie to their mothers.

They are the type of people that would lie to their mothers.
("That" is OK here because it is a class or type.)

Some teachers also tell you that that should be used with restrictive modifiers and that which should be used with nonrestrictive modifiers.

Examples:

The tall chair that sat in the guest bedroom was a priceless antique. ("that sat in the guest bedroom" refers to the "chair" and is a restrictive modifier, meaning that its use restricts the meaning of the sentence to a specific chair)

The tall chair, which sat in the guest bedroom, was a priceless antique. ("which sat in the guest bedroom" is nonrestrictive because it's not essential to the meaning of the sentence)

Historically, there is little evidence that this "rule" ever had a significant effect on English expression, but writers should be aware that some correspondents have been taught this practice.

Sometimes using which for a restrictive modifier can make a sentence sound better, especially if the sentence already uses the word that.

Example:

That is a book which I need for the class.

This "rule" can cause confusion with multiple clauses, questions, or certain constructions and compounds which use that. It is better to communicate more clearly than to worry about a questionable "rule."