what 是连接代词，不能引导定语从句，只能引导主语从句、宾语从句、表语从句等名词性从句。which是关系代词，可以引导定语从句，并在定语从句做主语或宾语。
We came to what was once a school.(宾语从句)
This is what we came here for.(表语从句)
What surprised me most was that he passed the driving test.(主语从句)
A plane is a machine which can fly.(引导定语从句并在句中做主语)
He was admitted to Beijing University,which made his parents very happy.(引导非限制性定语从句)
