很简单,从句做什么成分就是什么从句。比如I am a teacher.就是最简单的主语加系动词再加表语。不过这个表语teacher是个名词。再比如This is what I want to tell you .(这就是我想告诉你的)这句中what I want to tell you 就是个从句,它作is的表语,它就叫表语从句。换句话说,系动词后面的表语如果是个句子,就叫表语从句。同理,比如What I want to tell you is real.中,what I want to tell you 做了主语了这时它就叫主语从句了。
明白了吗?