7.非限制性定语从句
　　非限制性定语从句修饰先行词或整个主句，起补充说明的作用，如果省略掉，原句句意仍保持完整。非限制性定语从句要用逗号与主句隔开，其引导词不能用that。非限制性定语从句属于正式语体。经常考到的非限制性定语从句有以下三种。

　　1) 由which、as引导的非限制性定语从句。which、as代表整个主句。
　　He said that he had never seen her before, which was not true.
　　China is still a developing country, which is known to all of us.

　　2) 由 “介词+关系代词”引导的非限制性定语从句。
　　He sent her a letter, in which he said that he was sorry for what he had done to her.
　　He failed to pass the exam, because of which his parents scolded him.

　　4) 由 “数词、代词或名词+of +关系代词”引导的非限制性定语从句。
　　He had three sons, one of whom was my son’s classmate.
　　There are about twenty students in this course, most of whom are freshmen.
　　They are two different words, the spellings of which are easily confused.