wasn't 的原形动词,could的否定式?it's的展开形式?

问题描述:

wasn't 的原形动词,could的否定式?it's的展开形式?

isn't, couldn't, it is

be not;could not;it is/it has

was not
couldn't(could not)
it is

isn't
could not
it is; it has