1.Ask him which he wants.
* ask 是及物动词,后面跟有双宾语
* him 是间接宾语
* which he wants 是宾语从句,充当 ask 的直接宾语,.
2.The question is who can help us.
* is 是系动词
* who can help us 是表语从句.
3.What he says sounds reasonable.
* What he says 是主语从句
* sounds 是系动词
* reasonable 形容词作表语
本句意思是：他所说的听起来合情合理.
4.I have no idea what I should do.
* 句子的主要成分：I have no idea ,主谓宾结构
* what I should do 是同位语从句,用来进一步说明抽象名词 idea 的具体含义