The key to choosing between that and which is
knowing whether the clause to be introduced is essential
or nonessential: Essential clauses limit-or more
narrowly define-the meaning of the antecedent and are necessary
to the meaning of the sentence.
of essential clauses:
The car that I bought yesterday is missing.
(the receiver needs to know which car)
The lake that we visited last year is now
severely polluted. (identifies which lake is polluted)
The teacher who wrote the textbook is Mr.
Allen. (identifies which teacher)
clauses give additional or supplementary information about
the antecedent and could be removed from the sentence without
changing the meaning of the sentence.
Examples of nonessential clauses:
My new car, which I bought yesterday, is
missing. ("which I bought yesterday" could be eliminated
from the sentence without changing the meaning of the sentence)
Snowflake Lake, which we visited last year,
is now severely polluted. (additional information only-is
not needed to identify which Snowflake Lake)
Our teacher, who wrote the textbook, is Mr.
Allen. (additional information- "our teacher" is not identified
or differentiated by "who wrote the textbook")