限制性从句定语从句分为限制性定语从句和非限制性定语从句，非限制性定语从句在句中主要起补充说明作用，与主句之间往往用逗号隔开，独立存在，缺失也不会影响整个句子的意思。
1.who引导的非限制性定语从句在从句中做主语。
Our guide,who was a Canadian,was an excellent cook.
2.whom引导的非限制性定语从句，关系代词whom用于指人，在从句中作动词或介词宾语。
Peter,whom you met in London,is now back in Paris.
3.whose引导的非限制性定语从句，whose是关系代词who的所有格形式，在从句中作定语。whose通常指人，也可指动物或无生命的事物。
The boy,whose father is an engineer,studies very hard.
4.which引导的非限制性定语从句，关系代词which在非限制性定语从句中所指代和修饰的可以是主句中的名词、整个主句，在从句中作主语、宾语或表语。
These apple trees,which I planted three years ago,have not had any fruit.  He failed in the exam,which made his mother very angry.
在那么多人面前他有点紧张，这是可以理解的
5.when引导的非限制性定语从句 ，关系副词when在非限制性定语从句中作时间状语，指代主句中表示时间的词语。
He will put off the picnic until May 1st,when he will be free.
6.where引导的非限制性定语从句 ，关系副词where在非限制性定语从句中作地点状语，指代主句中表示地点的词语。
They went to London,where they lived for six months.
7.as引导的非限制性定语从句 ,as在非限制性定语从句中作主语、表语或宾语，且引出的从句位置比较灵活，可位于句首或句末，也可置于主句中间，通常由逗号将其与主句隔开。as有“正如……，就像……”之意。
As is known to the Americans,Mark Twain is a great American writer.  He is absorbed in work,as he often was.
As we all know,the earth is round.
8.“介词+关系代词”引导的非限制性定语从句 ，关系代词有时并不代表主句中某一确定的词，而是概括整个主句的意思。介词的选择取决于它与先行词的搭配或与从句中谓语动词的搭配。
They thanked Tom,without whose support they would not have succeeded.
9.“名词/代词+of+which/whom”引导的非限制性定语从句
She has two sons,both of whom are doctors.
10.非限制性定语从句引导词的特殊情况
(1)非限制性定语从句不可用that引导。
(2)关系代词在非限制性定语从句中作宾语时不可省略，若指人时，只用whom，不用who。
York,which I visited last year,is a nice old city.
(3)非限制性定语从句不可用why引导，需用for which替代why。