二.情态动词--must

1. must的词义
must表示"必须"或"应当"。如：
I must leave at 9:00我必须在九点钟离开这里。
Soldiers must obey orders.士兵必须服从命令。
You must get to the stallion before three o'clock. 你必须在三点钟以前到达车站。
You must come earlier tomorrow.你明天得早点来。

2. must的否定式
must的否定形式must not表示"不应该"或"不许可"，语气比较强烈。如：
We mustn't waste our time. 我们不应该浪费我们的时间。
Passengers must not walk across the railway line.旅客不要横穿铁路。
You mustn't miss that modern ballet，It is extremely good. 你可不要错过这个现代芭蕾舞剧，它好极了。
One must not divorce oneself from the masses.不应该脱离群众。

[注]说"不必"须用need not。如：
Must we hand in our exercises today? －No，You needn't.我们必须今天交练习吗? --不，不必今天交。

3. must也可表示过去
must只有现在式，通常用以表示现在，但有时也可以表示过去(多用在间接引语中)。如：
He told us we must all be ready by nine.他告诉我们在九点钟以前都得准备好。
It was too late to go back，we must go on.太晚了，回不去了，我们只有继续往前走。

[注] have to (必须)则可以表示各种时间。如：
I am afraid You will have to wait a while.我看你得等一会儿。At every step I had to pull my legs out of the snow.我每走一步都得将双腿从雪中拔出来。
 

4. must也可表示说话人对事物的推测
但比may肯定得多，相当于汉语的"一定"或"准是"。如：
He must be in the library now. 他现在准是在图书馆。
The old man must be over seventy now.那老人准有七十多岁了。
She must know how to do farm work. 她一定懂得怎样干农活。
You ate very little at breakfast today. You must be hungry now.
你今天早餐吃得很少，现在一定饿了。

[注] must加动词原形的进行式，也可表示对现在发生的动作的推测，有"一定"、"准是"的意思。如：
She must be working on the experimental plot.她现在一定在试验田里工作。

5. must加动词原形的完成式
说明对过去事物的推测，表示"一定"或"准是"的意思。如：
I mailed the letter two weeks ago. She must have received it.
我是两星期以前发的信，她一定已经收到了。
She must have studied English before. 她以前一定学过英语。
How did you know about it? Somebody must have told you. 你怎么知道这事的?一定有人告诉你了。

三.情态动词--may, might

1. may表示"允许"或"请求"
May I come in? －Yes，do. 我可以进来吗?--请进。
May I borrow you raincoat? 我可以借用你的雨衣吗?
You may go now. 你现在可以走了。
You may ring us up any time during office hours.在办公时间你可以随时给我们打电话。

[注] may表示"允许"的否定形式是must not(不应该，不许可)。如：
May I take this book out of the reading-room? －No，you mustn't. 我可以把这本书拿出阅览室吗? --不行。

2. may还可表示说话人的猜测，认为某一事情"或许"或"可能"发生
We may call on you this evening.我们也许今天晚上来看你。
She may not go to the concert tonight.今晚她可能不去听音乐会了。
The news may or may not be true. 消息也许是真的，也许不是真的。

[注] may用在感叹句中可表示祝愿，愿望如：
May you succeed.祝你成功。
May you have a pleasant journey.一路平安。

3. might为may的过去式
The speaker said we might ask him any question.作报告的人说我们可以向他提任何问题。
He said he might go to the Palace Museum on Sunday. 他说他星期天可能去参观故宫博物院。

[注一]night也可代替may，表示现在，但语气较为婉转客气或更加不肯定。如：
He night not come today.今天他也许不来了。
Might I make a suggestion? 我可以提个建议吗?

[注二]might用来表示现在时，还可表示"规劝"，如：
You might，pay more attention to spoken English.你要更多地注意英语口语。
You might shut the windows. The wind is blowing so hard outside.关上窗户吧。外面风很大。

4. may (might)后加动词的完成式
表示对过去的推测，认为某一事情在过去"可能"发生。这里may和might都指过去，不过might较为含蓄委婉或更加不肯定(常用于肯定结构)。如：
Nick may (might) have gone to the library.尼克可能到图书馆去了。
Our manager may (might) have gone to Harbin last weekend.
我们的经理上周末可能已经去哈尔滨了。
She may (might) have missed the plane.她也许没赶上飞机。

[注一] may和might常用在so that和in order that所引导的目的状语从句中(现在英语中还经常用can)。如：
Write is in simple language in order that everybody may understand it. 为了大家都看得懂，你要用简明的文字写。
He died，so that others night 1iye.他为了别人而牺牲了。

[注二] might加动词的完成式，可说明某一事情在过去没有实现并含有"劝告"甚至"责备"的意思。如：
You might have told me earlier.你本来可以早点告诉我的。
You might have been more careful.你本来可以多加小心。