Question

主语是复数时，谓语动词用单数。主语是单数时谓语动词用什么?

Answer 1

非谓语动词作主语时谓语用单数.但是如果是两个并列的就象是两个名词并列时候就用复数了.给你两个例子吧
To learn english well,listening is very important.
listengling作主语,动词用is
To learn Enlish well,listening and speaking are very important.
这句话中主语是listening和speaking,所以用了动词are

Answer 2

主谓必须一致。主语为单，谓语也单，vice versa。