When people use words and expressions ___from "standard language",it is called a dialect

题目

When people use words and expressions ___from "standard language",it is called a dialect
这句话是书上的,但有一点不懂 ____ 为什么不是are different 是different ,这点不太懂.

答案

因为这是一个When引导的从句,所以不能用are.