extremely important or urgent

[ + that ] The president said it was imperative that the release of all hostages be secured. 总统说当前要务是确保所有人质都获释。

[ + to infinitive ] It's imperative to act now before the problem gets really serious. 必须现在就采取行动，而不是等到问题变得难以收拾。

used to describe the form of a verb that is usually used for giving orders

In the phrase "Leave him alone!", the verb "leave" is in the imperative form. 在句子 Leave him alone! 中，动词 leave 用的是祈使语气。

the form of a verb that is usually used for giving orders

In the phrase "Leave him alone!", the verb "leave" is an imperative/is in the imperative. 在句子 Leave him alone! 中，动词 leave 是祈使语气动词。

something that is extremely important or urgent

Getting the unemployed back to work, said the mayor, is a moral imperative. 市长说，让失业人员重返工作岗位是一件道义上必须做的事。