I don't have to get up early. I have a day-off.

Why is it a mistake if I say: I mustn't get up early?

  • 1
    I Do not have to, means It is not required of me. I must not, means, It is forbidden for me (or by me).
    – EllieK
    Mar 3 '21 at 13:25

I mustn't get up early suggests that someone has told you "You must stay in bed late tomorrow". Obviously this isn't the case; it's just that you may do so if you want to.

I don't have to = it will be OK if I don't.

I must not = it will not be OK if I do.

  • @EllieK Well spotted - corrected. Mar 3 '21 at 13:43

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