should的否定形式是shouldnt,意为不应该,后一般接should(shouldnt)do sth;

must的否定是mustnt.但语气强烈,表禁止,

need的否定是neednt。

相关介绍

It's very important to me that you should be there.

你应该到场，这对我很重要。

By that time the new system should be up and running.

到那时这个新系统应该会运转起来了。

Give me one good reason why I should help you.

我为什么要帮你？给我一个充分的理由。