In this English lesson we’re going to discuss the difference between the words which and that. Both words can be used as relative pronouns to introduce adjective or relative clauses. Whether you should use which or that in this particular role can be a confusing concept for native and non-native English speakers alike. The general rule is that you should which in situations where the clause that it introduces is not essential to the meaning of the sentence. If the clause is essential to the meaning of the sentence, you should use that.
When to use “Which” and When to Use “That”
Which is usually used to introduce a nonrestrictive (unnecessary) clause. This type of clause is not essential to the meaning of a sentence. It merely provides additional information. Nonrestrictive clauses should be set off by commas. Thus, a which clause is one that provides nonessential information and is framed by commas:
The English Island, which offers both one-on-one and small group English classes, is located in Atlanta.
If you were to replace which with that in this example, it would probably “sound” wrong to a native English speaker. He or she would likely recognize that something is off about the construction of the sentence, even if he or she couldn’t articulate exactly why:
The English Island that offers both one-on-one and small group English classes is located in Atlanta.
So what, exactly, has happened here that would bother a native English speaker? Well, replacing which with that in this example changes the meaning of the sentence. Taken literally, the use of that and a restrictive clause indicates that there are multiple English Islands and that the one offering individual and group classes is based in Atlanta. While it’s true that our English classes are one of our core services, they are not essential to the meaning of this particular sentence. Forcing them into a restrictive (essential) clause via that only serves to make the meaning of the sentence less clear.
This is a very obvious example of how switching between which and that can completely change the meaning of a sentence. In other situations, the difference in meaning is less immediately apparent. This is usually the case when the subject is a generic noun, one that doesn’t identify a particular person, place, or thing. Consider the following examples where both which and that will sound equally “right” to a typical native English speaker:
Her house, which has green siding, needs a new roof.
She only owns one house, and that house needs a new roof.
Her house that has green siding needs a new roof.
She owns more than one house. The one with green siding needs a new roof.
The tree roots, which were just removed, had begun to damage our patio.
All the tree roots in our back yard had grown under our patio and caused the concrete to crack.
The tree roots that were just removed had begun to damage our patio.
We removed just the tree roots responsible for the crack.