[英语语法手册]情态动词--must
must
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。
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。
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。
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。
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。