When should "which" be used instead of "that"?

"Which" should be used when the clause is not essential to the meaning of the sentence, distinguishing it from "that," which is used for essential clauses. A non-essential clause, also known as a non-restrictive clause, adds additional information but does not change the fundamental meaning of the sentence. For example, in the sentence "The car, which is red, is parked outside," the phrase "which is red" provides extra information about the car but is not necessary to understand which car is being referred to.

In contrast, an essential clause, introduced by "that," is critical for clarifying the subject and would alter the sentence's meaning if omitted. This distinction is important in writing because it helps maintain clarity and precision.