定语从句什么时候用which、what等引导词

what不可以用在定语从句中。
定语从句的关系代词 that的用法: 若先行词指人或物则代替先行词在定语从句中充当主语或宾语

The animal that \which is lost is a panda.

先行词 指物代替先行词在定语从句中充当主语

She is the person that \who we are worried about.

先行词 指人代替先行词在定语从句中充当宾语

which引导的定语从句的用法
which可以引导定语从句，修饰表示事物的先行词， 在定语从句中充当主语、宾语或者表语。例如： He was reading a book， which was about war. 他正在读一本关于战争的书。（关系代词which在修饰book的定语从句中充当主语。）
He was reading a book， which he had bought from London. 他正在读一本书，这本书是从伦敦买回来的。（关系代词which在修饰book的定语从句中充当宾语。）
which引导非限定性定语从句时有一个特殊用法，即它所引导的非限定性定语从句修饰主句中更多的部分甚至整个主句（这时被定语从句所修饰的“先行词”要作广义理解）， which仍在定语从句中作主语、宾语或表语。例如：
He set free the birds happily， which was a celebration for his success. 他开心地把鸟放了，这是对他成功的一种庆祝。
把非限定性定语从句“which was a celebration for his success”的先行词视为主句中的“the birds”显然不符合整句的语境， 被“which was a celebration for his success”所修饰的是整个主句， which所引导的这类特殊的非限定性定语从句对主句所叙述的情况进行某种意义的补充说明，which常可译为“这一点，这件事”。

That:1、that只用在限定性定语从句中，非限定定语从句中不用that，介词后不用that，用which。2、限定性定语从句中，which和that在代替物时，一般可以通用。which和what，一个是关系代词一个是关系副词。如果从句的句子完整，那么就要用关系副词，不完整的话关系代词一般做从句的宾语。在非限定性定语从句中只宜用which不用that。一般有个窍门，如果前面有逗号隔开，一般都用whichwhere在句中作地点状语，代指地点

修饰地点的时候用where,如this is the place where i've stayed before.修饰物体的时候用which,如i love the watch which he gave me.没有名词的时候用what,如this is what i want.that基本可以替代各种情况，如this is thae place that i've stayed before.i love the watch that he gave me...this is the thing that i want.