must用法很多，今天只讲解must用在肯定句中表示比较有把握的推测，意为"一定，肯定，必定"。
（1）must表对现在的状态或现在正发生的事情的推测时， must 后面通常接系动词be 的原形或行为动词原形或进行式。
You must be hungry after all that walking.
走了这么远的路，你一定饿了吧。
There must be a way around the problem.
肯定有解决这个问题的办法。
You must be the happiest woman on the earth.你一定是世界上最幸福的女人。
The two boys look very much alike.They must be twins.
这两个男孩长得很像，一定是双胞胎吧。
At 29 Russell must be one of the youngest ever Wembley referees.
29岁的拉赛尔一定是有史以来温布利球场上的裁判员中最年轻的一个。
You have worked hard all day.You must be tired. 你辛苦干一整天，一定累了。
He must know the result.
他一定知道结果。
Mum must be cooking supper now.妈妈现在一定在做晚饭。
He must be working in his office. 他一定在办公室工作呢。
比较：
He must be staying there.
他现在肯定呆在那里。
He must stay there.
他必须呆在那。
（2） must 表示对已发生的事情的推测时，must 要接完成式。
He must have known what she wanted.
他一定早已知道她想要什么了。
He must have finished his work.他一定完成他的工作了。
You must have mistaken my meaning.
你一定误会了我的意思。
I didn't hear the phone. I must have been asleep. 我刚才没有听到电话，我想必是睡着了。
（3） must表示对过去某时正发生的事情的推测，must 后面要接不定式的完成进行式。
---Why didn't you answer my phone call?
---Well, I must have been sleeping, so I didn't hear it.
为什么你没有接电话？哦，我那时一定是睡着了，所以没有听见。
must表示推测时，指有把握的、有根据的推测，只能用于肯定句。可以对现在（注：对现在情况的推测用“must+动词原形”）、过去（注：对过去情况进行推测时用must have done）或将来的情况进行推测。
其后接的动词一般是be ,have ,know等状态动词，如果是动作动词要换成其他的表达法，如：
Don’t worry,he is sure to come.别急，他一定会来的。
我们队肯定会赢的。
Our team is sure to win.
Our team is certain to win.
Our team is bound to win.
Our team will surely win.
Our team will certainly win.
I’m sure that our team will win.
It’s certain that our team will win.
It’s sure thing that our team will win.
如果硬要说：Our team must win.则含义为：我们队必须（要）赢。
（4）否定推测用can't。
If Tom didn't leave here until five o'clock, he can't be home yet.
如果汤姆五点才离开这儿，他此时一定还未到家。
It can’t be ours.这个不可能是我们的。