为什么是It is imperative that this mission not fail.而不是It is imperative that this mission don't fail.

Imperative “必要的,不可避免的；命令的,势在必行的"像这样的词,后面接的句子谓语为“should + 动词原形”其中,should可以省略.句子结构为：it's imperative that sb + (should)+ 谓语动词原形+其他成分 eg. 1. It ...It is imperative that this mission don'tfail.那这么说对不对It's essential thatthis mission not fail.be essential that 用虚拟语气是固定搭配还是这种BE+形容词+虚拟语气的句式都是这样的在主语从句中,谓语动词的虚拟语气结构用"should+动词原形"的结构,表示惊奇、不相信、惋惜、理应如此等。例如:It is necessary ( appropriate, crucial, desirable,dreadful,essential,important,improper,incredible,indispensable,insistent,natural,preferablepossibleprobableproperrightsignificantsurprisingstrangetragicurgentvital wrong etc.) that we should clean the room every day. It was a pity(a shame,no wonder,etc.) that you should be so careless. It will be desired(suggested,decided,ordered,requested,proposed,etc.)t hat she should finish her homework this afternoon. 在上述三种主语从句中,should意为"应该"、"竟然",可以省去,但不可换用为其他形式。