Must - Free english lesson
Chapter The modals

Rule : « Must » cannot have a past tense form.
We use « must/mustn't » in order to translate an obligation or an interdiction.
You must stop this mess.
He musn't watch television.

We also use « must » to indicate a very strong probability.
It must be very nice.
He must be ill.

As we have specified in the grammatical properties, « must » has not a past tense form. In a past sentence, we will use « had to » in order to indicate an obligation.
He had to go to an interview.

In order to express an obligation, we can also use « have to ».
We use « have to » when the speaker is not the source of this obligation.
You have to clean your room. In this case, we inform the person that he/she has the obligation to clean the room, but we are not at the origin of this obligation.
On the other hand, in the sentence « You must clean your room », it is the author of the sentence that gives the order.

Be careful, « mustn't » and « don't have to » do not have the same meaning.
« Mustn't » expresses an interdiction and « haven't to » indicates that we are not obliged to accomplish an action. You mustn't touch the paints.
You don't have to go to school.

Progress in english with the story of the Lacoste family.
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