(1)应该(当)(用于表示义务或责任)(used to express the obligation or responsibility)：You～not do that. 你不应该那样做。S～we remain in the Commonwealth? 我们应该留在英联邦吗?They～tell us. 他们应该告诉我们。

(2)应该(当)(用于表示征求建议或准许)(used to ask for advice or permission)：S～I turn off the light? 我要不要关灯? Where～we meet tonight? 我们今晚在哪里见面?

(3)应该(用于表示建议)(used to express one's advice)： Crimes～be punished. 犯罪应该受到惩罚。The birds～n't be in a cage. 鸟不应关在笼中。You～stop gambling/smoking. 你不应该再赌博/抽烟了。

(4)该是，可能(用于表示试探性的推断)(used to express a tentative conclusion)： There～n't be any difficulty，I think. 我想不应该有任何困难。She～be alone. 她可能是一个人吧。She left yesterday，so she～have arrived. 她是昨天动身的，按说应该已经到了。

(5)就……(用于表达假定的结果)(used to express the unreal consequence)(fml)：If I knew that I～tell you. 我要是知道就告诉你了。Were he to arrive， I～be pleased. 他要是能到，我就高兴了。 I～not have won without his help. 要是没有他的帮助，我不会赢。

(6)竟然(used in a that-clause after certain adjectives)： It's strange (odd) that you～come today/mention that. 真奇怪，你竟然今天来了/提起那件事。 I am very sorry that you～feel unhappy. 你竟会感到不高兴，我非常遗憾。He was anxious that the plan～not fail. 他切盼着这个计划会成功。

(7)万一(used to express sth might，but not likely to happen)(用于if，in case 后，或当主谓语倒置)：If you～make it known，it'd better let me know first. 万一你想让人知道这件事，最好先告诉我。S～you come，you will be warmly welcomed. 你要是来，会受到热烈欢迎的。If the report～not have been finished in time，the meeting will certainly be put off. 万一报告不能如期完成，会议当然得延期举行。

(8)以便(表示目的或动机)(used to express purpose)(用于so that， in order that 后)(fml)：The secretary copied the report so that each one～have one. 秘书复制了这份报告，以便大家人手一份。The guide marked the map in order that the visitor～know the destination. 导游在地图上作了标记，为的是让游客知道哪里是目的地。

(9)倒，可以(表示客气请求)(used to make polite requests)： I～be delighted to try it. 我倒是喜欢试一试。There are a lot of things I～have asked you. 有许多事我本想问你的。

(10)可能，会(表示不成熟的看法)(used to express one's tentative opinions)： I～think it was about twelve years ago. 我想那该是大约12年前吧。I～say he is 140 pounds in weight. 我说他的体重该有140磅吧。 “Are these enough?” “I～think so”. “这些够了吗?” “我想是够了”。I～think that he would apologize. 我想他会去道歉。

(11)怎么，竟然(used to express one's doubt or surprise)： Why～they think in that way? 他们怎么会那样想呢?How～he know me? 他怎么会知道我呢?I turned around in the street and who～be walking behind me but one of my old schoolmates. 我在街上转悠，谁料想跟在我身后的竟然是我的一个老同学。

【辨异】should 与have、have to、need、must和ought to 的区别见HAVE。