How do we use the modal verb "must" in these contexts?

[amna]

pl explain me the use of must,

IT must have rained.

She is speaking well, she must practice it.

U must not drop it.

U must do it

pl help me

AMNA

[Torsten]

[amnass]

Thanks,

please explain what is the difference between Have to do and Must do, which should be used in conversation & writing

Thanks again

Amna

[Torsten]

Must is very strong and often means you have no alternative.

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