Have to or must?

[Arlindomarco]

Use "must" for strong obligations at the moment of speaking.
Use "have to" for daily responsibilities.
Examples:
I have to catch a train to work every morning.
I must get going.It's late!
Happy New Year!

[Mister Micawber]

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This is not a good 'rule', arlindomarco. HERE is a better explanation.
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Native English teacher at Mister Micawber's

[Alan]

Hi arlindomarco,

Another difference between the two constructions is that 'must' is very often external compulsion and 'have to' is internal compulsion. In other words 'must' is often another person telling you what to do and 'have to' is often you telling yourself what to do. For example:

I must be early for work tomorrow because my boss has told me to be and so I have to get up early.

Alan
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English as a Second Language
You can read my ESL story Phrasal Verbs/look