Many
students get confused between the relative pronouns “that” and “which”.
Although some people think that those two are interchangeable, they actually
are used in completely separate cases. The rules for their usage are as
follows:
·
"That" is used when the noun it is relating is being
specified out of many.
·
"Which" is used when the noun it is relating is having
more information added to it by the phrase or clause after the relative pronoun
“which”.
Let's look at the following examples:
"The car that the police found yesterday contained enough evidence
to put the thief in jail."
"Those apples, which
my aunt brought yesterday, are beginning to rot."
In
the first example, "that" is specifying one particular car: the car
that the police found yesterday. There are many cars in the world, but the one
the police found yesterday is the one we are talking about. “That” allows the
reader to understand this point. Information after the relative pronoun
"that" is always necessary to the sentence. There is no need to use commas
to separate this information from the rest of the sentence.
In
the second example, "which" is signaling the beginning of extra
information about the apples. It would be fine to say, "Those apples are
beginning to rot." The information that follows “which” is just adding an
extra thought that the reader doesn’t necessarily need to know. When using
"which", always separate the extra information that follows with
commas. This is similar to an appositive except for the fact that an appositive
doesn’t use a relative pronoun.
In
conclusion, when you want to add vital information that is related to a
specific noun, use the relative pronoun "that"; don't add any commas
here. On the other hand, when you want to add extra information to a sentence
that is related to a noun, use the relative pronoun "which" and add
commas to separate the extra information.