Which, Who, and That

who is the personal form of which. who is not a valid replacement for that.

who and which are used for parenthetical clauses, which are separated from the rest of the sentence by commas. that is used for restrictive clauses, which change the meaning of the word that precedes them.

For example, compare the effect of which and who with commas, against that with no comma:

In the first case, the parenthetical which and who clauses provide additional information but do not change the fundamental meaning of the sentence: The cookies are for the boys. The writer or speaker assumes that there is no ambiguity about what is meant by the cookies and the boys, and is simply supplying additional information.

The first sentence could be rewritten as three separate ideas:

The second case could not easily be rewritten as separate sentences, as it is all one single idea. The restrictive thats don't simply supply additional information; they affect the meaning of the sentence. The room could have other cookies that are not under consideration. There could be boys that aren't hungry and so don't get the cookies that are on the table. Or there could be girls, who might or might not be hungry, that are entitled to something else.