"must" and "have to"

[pink dragon]

I’m not sure when to use these words.
I have read somewhere that “must” is used when you want it, and “have to” is used when you can’t avoid it. Is that true?
For example, which should I use when I want to say the following?
“Please tell Kathy I miss her so much. She must/has to come with you next time”.

[Tdol]

I'd use 'must' there to express how much I want her to be there.

[pink dragon]

Thank you, tdol!

What does it sound like if I use “has to” there?
Will the meaning of the sentence change?

[SweetMommaSue]

Originally Posted by pink dragon

Thank you, tdol!

What does it sound like if I use “has to” there?
Will the meaning of the sentence change?

Hello pink dragon,
To my American ears, they sound the same. They are virtually interchangeable in AE. In some instances, "must" may sound a little more formal, but really, there is no change in meaning.

Smiles,
SweetMommaSue

[Tdol]

They're pretty interchangeable in BrE to. When urging people, we tend to use 'must', but 'have to' would not be incorrect.

[rajan]

have to is used for external compulsions.

must for personal obligations.

Originally Posted by tdol

They're pretty interchangeable in BrE to. When urging people, we tend to use 'must', but 'have to' would not be incorrect.

[Tdol]

That doesn't mean that there are not many cases where they can be used interchangeably, and even your distinction is less clearcut in practice because we also use 'must' to express true asbsolutes, many of which are external.

[pink dragon]

Thank you very much, SweetMommaSue, rajan, and tdol!