I suppose I must get a burglar alarm, too.

[Couch Tomato]

Where necessary, make corrections in the underlined parts of this email message.
[...]
I suppose I may must get a burglar alarm, too. I must say I've never really wanted one, but needs must!
(Advanced Grammar in Use, Martin Hewings)

They key suggests "I suppose I may have to get..." but can we also say, "I suppose I must get..."? Is it wrong because in the sentence the speaker is expressing doubt by using "suppose"?

Thank you in advance

 

[Bevj]

In my opinion both are correct.
'I must get...' sounds more definite than 'I may have to...', not because of 'suppose' but because of the use of 'may'.

 

[Biffo]

I think that the exercise is to detect an error in "may must get" and then change it to the nearest valid equivalent. This can be done by changing 'must' to 'have to' and leaving the other words as they were.

 

[Couch Tomato]

Thanks, Bevj.

I think that the exercise is to detect an error in "may must get" and then change it to the nearest valid equivalent. This can be done by changing 'must' to 'have to' and leaving the other words as they were.

Yes, that's probably it.

 

[Myridon]

It seems to me that "may get" and "must get" are both correct.