what不可以引导非限制性定于从句
1．限制性定语从句说明先行词的情况，对先行词起限定作用，与先行词关系十分密切，不可用逗号隔开，也不可省略，否则全句意义就不完整.
如：
This is the telegram which he refers to.
Is there anything (that) I can do for you?
2．非限制性定语从句只是对先行词作补充说明，没有限定作用，它与先行词的关系比较松散，因而不是关键性的，如果省略，原句的意义仍然完整.这种从句在朗读时要有停顿，在文字中通常用逗号与主句隔开.
如：
This note was left by Tom，who was here a moment ago.
As a boy，he was always making things，most of which were electric.
引导非限制性定语从句的关系词有：who，whom，whose，which，when 和 where，不可以用that和why.
另外，非限制性定语从句从意义上讲，相当于一个并列句，在口语中使用并不普遍，在日常生活中，人们通常用并列句或简单句来表达.
如：
I told the story to John，who later did it to his brother.
= I told the story to John，and he later told it to his brother.
Yesterday I happened to see John，who was eager to have a talk with you.
= Yesterday I happened to see John and he was eager to have a talk with you
非限制性定语从句不可以用what引导，what引导的是一般是名词性从句。非限定性定语从句起补充说明作用，缺少也不会影响全句的理解，它与主句往往用逗号隔开，如若定语从句有限制性和非限制性两种。限制性定语从句是先行词不可缺少的部分，去掉它主句意思往往不明确非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开，将非限定性定语从句放在句子中间，其前后都需要用逗号隔开。非限制性定语从句的引导词有：who引导，whose引导，which引导，when引导，where引导，as引导。
