They spent as many hours as they could -----their lessons

题目

They spent as many hours as they could -----their lessons
为什么是填to study,而不是根据spend sth on sth,spend sth (in) doing sth,填studying为什么不对?

答案

to study这里跟词组没关系.动词不定式表示目的.
他们花尽可能多的时间 来学习课程.来学习课程是目的,用to study表示