spend on doing sth.指花费金钱做某事;

spend on sth. 指花费金钱.时间在某事上;

spend in doing sth.指花费时间做某事.

I spent what little time I had with my family.

我仅有的一点儿时间都和家人在一起度过了。

The money could be better spent on more urgent cases.

这笔钱用于较紧迫的事情也许会好些。

I spent most time on the first question.

我在第一个问题上花的时间最多。