But we couldn't find them all为什么不是found

But we couldn't find them all为什么不是found
But we couldn't find them all
为什么不是found

But we couldn't find them all为什么不是found
couldn't后要接动词原形
翻译：但是我们怎么也找不到他们.

前面有could，后面+动词原形

因为前面是情态动词could

因为情态动词 could后接动词原形。