should是情态动词，它也是shall的过去式。其用法较多，主要如下。
一、用于纠正别人的错误，批评错误行为。意思是应该，应当。
You shouldn’t be late, for this meeting is very important.你不该迟到的，这个会议非常重要。
You should have been faster.你应该快速一点。
二、提出或寻求建议，意思是该，可以。
Should I call him first and apologize?我是不是应该先打电话给他道歉？
If I were you, I should carry out the project whatever others say. 如果我是你的话，我就会完成这个工程，不管别人说什么。
三、表示预期，期待某事会发生或成真。
They should have finished their work by thia Friday.周五它们应该完成他们的工作了。
The weather should be fine tomorrow.明天天气应该很好。
四、表示出乎意料，万一，假如。
If you should change your mind, call this number.如果你改主意了，请拨打这个电话。
If the bike should break in the first year, we will give you a new one如果这个自行车第一年坏了，我们给您一辆新的。
五、本应，本当（表示与事实相反）
It should be sunny today, according the weather forecast.根据天气预报，今天天气应该晴朗才是。（其实今天天气不好）
You should have closed the windows before you leave the room.你离开房间之前，应该把所有的窗户关掉才是。（其实，你没有关窗）
六、用于表达建议、命令、请求、决定、重要性等词后面。
It is necessary that we should follow the doctor’s advice.
The teacher suggested that Tom should finish the task by himselves.
七、作为shall的过去式，表达过去将来。
I said that I should be glad to help.我说过我会很乐意帮忙。
八、虚拟语气中，表达虚拟动作。
If somebody should call me, please tell him that I am attending an important meeting.
Should somebody call me, please tell him that I am attending an important meeting.
两句话意思完全相同。第二句是把第一句的if省略，should提前的用法。这是虚拟语气特有用法，具体情况另文详述。
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