I know I must, but do I have to? I know I have to, but must I?

[Glizdka]

These are my own sentences. I'm trying to find a sentence that directly shows the difference between have to and must.

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A) "I know I must, but do I have to?"
B) "I know I have to, but must I?"

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I want the sentence to mean, roughly, "I know it's needed/expected/the right thing to do, but is it really necessary/mandatory/am I obliged to do that?"

I know there's quite a difference between how must is used in AmE and BrE, so I'd like to hear from both sides of the pond.

I'd say B is the sentence I'm looking for.

 

[GoesStation]

We rarely use must that way in American English.

 

[emsr2d2]

For me, both sentences are meaningless. "have to" and "must" mean the same. Something that's "needed/expected/the right thing to do" is expressed with "should".

He knows he should drive below the speed limit but does he have to?
I know I should take the dog for a walk but must I?

It should be noted that we don't use "must" in the interrogative very often.

 

[Glizdka]

Should I scrap the idea, then?

 

[emsr2d2]

I would.

 

[GoesStation]

Should I scrap the idea, then?

You must have dozens of better ones percolating away in your fertile imagination.