非限制性定语从句是相对于限制性定语而言的.限制新性定语从句的修饰语必须紧跟在被修饰的词后,去掉这些修饰性成分句子就显得表意不明.如He is a boy ( who is very clever).若去掉括号中的就显得奇怪了.而非限制性定语从句的修饰语与先行词以逗号隔开,且删掉后不影响句意的表达.有如下几种形式：As was expected,we finally reached home.（由As引导的非限制性定语从句,修饰整个句子,表达情感）He failed the exam,which was very annoying.（which修饰前面整个句子）l love the girl,who is so kind and beautiful.（who 修饰 girl)
就是总结一下,有两个不同处.删去有无影响,先行词可否为一个句子和简单的名词,代词类.