used to show that it is necessary or very important that something happens in the present or future

Meat must be cooked thoroughly. 肉必须完全煮熟。

I must get some sleep. 我必须睡会儿觉了。

You mustn't show this letter to anyone else. 你不许把这封信给其他人看。

Luggage must not be left unattended (= it is against the rules). 行李不能无人看管。

formal Must you leave so soon? 你非得走这么早吗？

formal "Must I sign this?" "Yes, you must." “我必须在这上面签字吗？”“是的，必须。”

If you say that you must do something, you mean that you strongly intend to do something in the future.

I must phone my sister. 我一定要给我妹妹打个电话。

We must get someone to fix that wheel. 我们一定要找人修修那个轮子。

UK I mustn't bite my nails. 我一定不咬指甲了。

used for emphasis

I must say, you look absolutely great. 我得说，你看上去好极了。

I must admit, I wasn't looking forward to it. 我必须承认，我并不是很期待这件事情。

If you tell someone else that they must do something pleasant, you are emphasizing that you think it is a good idea to do that.

You must come and stay with us one weekend. 你一定要来和我们一起度周末。

We must meet for lunch soon. 我们一定要快快找个日子相约吃个午饭。

used to show that something is very likely, probable, or certain to be true

Harry's been driving all day - he must be tired. 哈里开了一天车，他一定累了。

There's no food left - we must have eaten it all. 没剩什么食物了，想必我们全吃光了。

When you got lost in the forest you must have been very frightened. 那天你在森林里迷路了，一定很害怕吧。

"You must know Frank." "No, I don't." “你一定认识弗兰克。”“不，我不认识。”

something that is necessary

If you live in the country a car is a must. 如果你住在乡下，汽车是必不可少的。

something that is so good, you must do it, have it, or see it

The cashmere scarf is this season's must-have. 羊绒围巾是本季的必备品。

It's a moderately entertaining film but it's certainly not a must-see. 这部电影娱乐性还不错，但肯定不是非看不可的。