the book is very hard to understand 为什么要用 to?

问题描述:

the book is very hard to understand 为什么要用 to?

to understand是不定式,动词understand在句中只能作谓语,如果想做动词的宾语就要加to,形成不定式.另外简单点说就是两个动词hard 和understand 不能连用,所以加to.