1.表语(一般是形容词,名词,跟在系动词(be)后面）
He is tall.(tall---表语)
It is water.(water)
2.表语从句(is + that/what/how/when/where 从句）当表语用的句子
The problem is (how/when to get there)
3.宾语从句( do + that/how）充当宾语的句子
I know (that she likes you)
4.同位语从句 （当同位语用的句子）
Tom ,(who is the monitor),is very rude.