主语从句做主语，主句谓语动词的单复数需根据从句表示的单复数意义决定。
weight什么时候用复数
1．what引导的主语从句做主语
（1）通常情况下，由what引导的主语从句放在句首时，其后的谓语动词，按语法一致的原则用单数。eg:What she said is correct.      What caused the accident is not clear.
（2）当主句中的表语是复数形式，或what从句的谓语动词及其补足语均为复数形式时，主句的谓语动词应用复数。  eg:What he gave me are five English books.
What are used as chief building materials today are steel and concrete.
（3）当what引导的主语从句是一个由and连接的并列结构时，应根据句意确定其表达的单复数意义，然后决定主句谓语动词的单复数形式。
eg:What he says and acts does not concern me.
What he says and acts do not agree.  What I say and what I think are no business of yours.
2．由其它词引导的从句做主语时，谓语动词用单数形式。
Whether they will hold a party or not has not been decided.
Who is responsible for the accidents is not clear.
3．当同一个主语从句是由两个不同的引导词引导时，因为它表达的还是同一件事，因此，谓语动词一般用单数。
eg:When and where we should hold the meeting has not been decided yet.
4．当主语是由两个由and连接的主语从句组成，通常表示的是两件不同的事情，主句谓语动词需用复数形式。
What you eat and how much you exercise are important factors in a weight loss program.
What caused the accident and who was responsible for it remain a mystery to us.