should是shall过去式，但当should本身做一个词的时候不能把它当作shall的过去式；should作独立的情态动词时候没有过去式和过去分词。

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。