同学，是定语从句的限定性和非限定性袄。

一、定语从句分为限制性和非限制性两种。限制性定语从句不可缺少先行词，去掉它主句意思往往就不明确了，它与主句之间通常不用逗号分开;非限制性定语从句是对先行词的附加说明，去掉它也不会影响主句的意思。如:Do you know the professor who is speaking at the meeting? (限制性定语从句) 

English is an important subject,which every student should study well. (非限制性定语从句)

二、 限制性定语从句的先行词往往是某一个词, 但非限制性定语从句的先行词可以是一个词， 也可以是句子的一部分,甚至是整个主句，此时非限制性定语从句一般由which或as引导， as引导非限制性定语从句常用在as is said /known / expected / mentioned / reported / announced等结构中。如:He has passed the college entrance examination, which makes his parents quite happy.As is known to everybody, the moon travels round the earth once every month.

三、关系词的使用情况有所不同1. that不可引导非限制性定语从句。如:Her father gave her a computer for her birthday, which pleased her a lot.2. why 不用在非限制性定语从句中，需用for which代替。如: | told them the reason, for which I didn't attend the party last week.3.关系代词替代情况不同关系代词whom在限制性定语从句中作宾语时可用who代替whom,但whom在非限制性定语从句中作宾语时不可用who代替。如:This is the girl whom who | have been in love with for six years.The young man had a new girl friend, whom he wanted to show all his love for.4.关系代词省略情况不同关系代词在限制性定语从句中作宾语时可以省略;非限制性定语从句的所有关系代词均不可省略。如:She is the new student whom / who / that | want to introduce to you.All the books there, which have beautiful pictures in them, were written by him.