See a brief explanation of the basic structure of all modal auxiliary verbs...
Must and have to express the same meaning. They are used to talk about strong obligation:
You must / have to lock the door when you come in at night.
They must / have to finish the job today.
The differences between must and have to is not meaning but usage. Must is used when the speaker gives a direct order. Have to is used when the speaker talks about another person's orders or about rules and regulations:
You must finish your homework by tomorrow. (Teacher to a child)
In England you have to drive on the left. (A friend to a visitor to the UK)*
*If the speaker said must here, it would sound like a direct order when what he means really is that driving on the left is a rule of the road in the UK. Students of English often use must too much so they sound like they are giving orders all the time!
The exception to this rule is when we use the negative. The negative of have to is don't have to. This does not talk about obligation; it means don't need to. For this reason mustn't is used both for giving orders or talking about rules.
In England you mustn't drive on the right. (and not, you don't have to drive...)
Must is very common in the first person (I, we) when we talk about obligations we impose on ourselves. Sometimes it sounds like the voice of our conscience talking:
I must get up early tomorrow or I'll be late again.
I must talk to Mary as soon as possible about our holiday plans.
We must ring the Jones's. We haven't seen them for ages.
If we need the infinitive form of these modals, we must remember that modal auxiliaries cannot be used as infinitives so we must use another verb of obligation that has an infinitive like "have to":
Paco used to have to go every weekend (and not, Paco used to must...).
I'll have to take the children to school tomorrow (and not, I'll should take...).
We use must to talk about assumptions (deductions). We make these assumptions from evidence we see, hear or that someone tells us. We distinguish this use of must from a meaning of obligation only by context:
He must be very strong to lift all that weight.
It must be about nine o'clock because it's getting dark.
John must be at home by now; he left here an hour ago.
She must be playing golf; her golf clubs aren't here.
We can make deductions about past events by using the perfect infinitive: must + have + past participle (or, must + have + been + "ing" form of the verb):
You must have worked a lot last week; you look very tired.
You must have eaten too much last night because now you feel ill.
He must have travelled a lot during his life because he knows a lot about the world.
He didn't answer the phone when I called. He must have been having a bath.
In all the above sentences, the speaker is saying what he thinks is true but without proof.
Explanation of modal auxiliary verbs in general...
Modal auxiliaries must, can't, may, could, might for deductions (listen and read)...
Modal auxiliary verbs 1 (can, could, may, might)...
Modal auxiliary verbs 2 (should, ought to)...
Modal auxiliary verbs 4 (would)...
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