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 that
s 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.