imperative 意思是必须，命令，这里it is imperative表示祈使句型，后面的从句要自动省略“should”实际是 It is imperative that this mission (should)not fail. 意思是“这次任务必须成功！”表示“命令、建议、要求”之类的动词或者名词接的从句（宾语或者同位语）都要省略should，比如：He suggested that we (should)go there.一般需要省略should的祈使动词有propose, suggest, (it is) imperative, command等。有问题请追加~...
imperative用作形容词的基本意思是“必要的,紧急的,极重要的”,在句中多用作表语,在“It is/was imperative that-clause”结构中从句的谓语动词多用虚拟式,英式英语中常用“should+动词原形”,而在美式英语中则常省略should,It is imperative that this mission don't ...
