改病句：the firemen could not found the people.

found→find

the firemen could not find the people.
could 后一定要加动词的原型，原句中为过去式found所以错。

情态动词（can, must,might 等等）后面加动词原形，所以原句是
the firmen could not find the people.

could是情态动词，后面用动词原型，所以把found改成find.