should是情態動詞，它也是shall的過去式。其用法較多，主要如下。
一、用於糾正別人的錯誤，批評錯誤行為。意思是應該，應當。
You shouldn’t be late, for this meeting is very important.你不該遲到的，這個會議非常重要。
You should have been faster.你應該快速一點。
二、提出或尋求建議，意思是該，可以。
Should I call him first and apologize?我是不是應該先打電話給他道歉？
If I were you, I should carry out the project whatever others say.如果我是你的話，我就會完成這個工程，不管別人說什麼。
三、表示預期，期待某事會發生或成真。
They should have finished their work by thia Friday.周五它們應該完成他們的工作了。
The weather should be fine tomorrow.明天天氣應該很好。
四、表示出乎意料，萬一，假如。
If you should change your mind, call this number.如果你改主意了，請撥打這個電話。
If the bike should break in the first year, we will give you a newone如果這個自行車第一年壞了，我們給您一輛新的。
五、本應，本當（表示與事實相反）
It should be sunny today, according the weather forecast.根據天氣預報，今天天氣應該晴朗才是。（其實今天天氣不好）
You should have closed the windows before you leave the room.你離開房間之前，應該把所有的窗戶關掉才是。（其實，你沒有關窗）
六、用於表達建議、命令、請求、決定、重要性等詞後面。
It is necessary that we should follow the doctor’s advice.
The teacher suggested that Tom should finish the task by himselves.
七、作為shall的過去式，表達過去將來。
I said that I should be glad to help.我說過我會很樂意幫忙。
八、虛擬語氣中，表達虛擬動作。
If somebody should call me, please tell him that I am attending animportant meeting.
Should somebody call me, please tell him that I am attending animportant meeting.
兩句話意思完全相同。第二句是把第一句的if省略，should提前的用法。這是虛擬語氣特有用法，具體情況另文詳述。