Should I Use “Must” or “Have to?”
The general rule for using must or have to to express an obligation is that must is used when the obligation comes from the speaker, and have to is used when the obligation comes from someone or something other than the speaker.
An example of the correct use of must would be when a parent tells their child, “You must finish your dinner if you want dessert.” The parent (that is, the speaker) has set this rule, so must is used.
An example of the correct use of have to would be if someone said, “I have to be at work at 9:00 tomorrow.” Someone other than the speaker has made this rule, so have to is used.
Additionally, in British English, it is common to use have got to interchangeably with have to. As such, “I have got to be at work at 9:00 tomorrow,” is perfectly acceptable in Britain, but is not used commonly in America.
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