How many roads must a man walk down...

(Bob Dylan)

This verse was made almost universally known by the Flower-Power movement (and I still tend to hum it when I am walking in an unknown town looking for a pub). It shows the word must in its most frequent function: to express obligation. As long as you use it like in this quotation, everything will be fine.

must modals

Unless... want to negate it.

A man must do what a man must do. An obligation is imposed by a higher power (like your teacher) and it has never been the plan to give you a choice. The English language just follows this path of logical thinking and does not offer you a way to negate must.

Should you still try to say I must not..., language will take revenge and use its most powerful and terrible weapon: Misunderstanding.
When must is used in combination with not, it will be understood as should not, may not or be not allowed to. So when you proudly assemble the following English statement of independence and sovereignty
I must not do homework,
you will not be saying I don't have to do ..., but I am not allowed to do homework.
So you will not sound like the strong guy, but like the weakling trying to suck up to his teacher. Is this the impression you want to make? No? Then always use

the negative form of have to

when you want to negate must.


I must read this book. I don't have to read this book.
You must read the manual before you can use this device. You don't have to read the manual before you can use this device.
She must be there before 9 o'clock. She doesn't have to be there before 9 o'clock.
A man must do what a man must do. (You mustn't change this sentence. Some things are just so undeniably true...)

You can find one more way to express the negated form of obligation under need from the modals table.

But of course you can actively use this effect...

... to express prohibition by saying things like You must not eat ice-cream. Try it out with your brother or sister, the little numbskull.
You often find this phrase in a passive voice construction, like in The newspapers must not be taken from the reception area. (as if you were interested in reading!)

You assume there is more to say...

...and you are right. Let's talk about your assumptions. This is a concept you need whenever you want to talk about something without knowing much about it. Then you don't know, but you assume. You can use the word must to express that. You must have understood by now, I assume.
If you haven't, here are some examples:

He must have seen the red light at the crossroads.

You think he saw it because it is so big and bright, but unfortunately you can't ask him any more.

It must feel great to have passed all exams.

Here you try to imagine a feeling you might experience yourself one day. (Mind you, I said 'might'.)

You must be crazy.

Sometimes we pretend to assume things we actually know for a fact.

It must be difficult to be so intelligent.

Oh yes, it is!

We started this page with a singer and we will finish it with another one. Here are some 'must' assumptions of Arlo Guthrie:

Santa Claus wears a red suit.
He must be a communist.
And a beard and long hair.
Must be a pacifist.
And what's in that pipe that he's smoking?

Here you can go back to the modals table.