主语从句,宾语从句,定语从句,状语从句,表语从句等.只起连接词作用,在句中不作成份时可省去.如宾语从句 I know (that)He has come back.中that只是引导宾语从句的引导词,就可以 省去.而定语从句 I don't know the man that spoke to me just now.中,that是先行词,不能省.