we had a hard time __________the solution to this problem

问题描述:

we had a hard time __________the solution to this problem
用find finding to find 还是found?
答案是finding
为什么呢?
为什么不是to find

finding 表示动作已完成,与过去式had呼应
to find 表示动作尚未发生,与had搭配不上