Relative Pronouns: Distinguishing
"That" from "Which"
It is important to distinguish between the
relative pronouns "that" and "which," probably
the most commonly misused relative pronouns. Generally
"which" is used where "that" should be used.
The relative pronoun "that" introduces an essential
clause (defining, limiting), whereas the relative pronoun "which"
introduces a nonessential clause (nondefining, descriptive).
Look at the examples below to examine their different uses.
Example of an essential clause
The rat that survived the treatment developed
The clause "that survived the treatment"
is essential because it defines the subject of the sentence.
Without it, the meaning of the sentence would be lost.
Example of a nonessential
The third rat, which survived the therapy,
The clause "which survived the therapy"
is descriptive; the subject has already been identified by the
word "third." In addition, nonessential clauses using
"which" are always enclosed by commas.
Now go to the next step to see how "which"
clauses are misused.