What's the difference between můžu and mohu? Or are they interchangeable ?
Completely interchangeable, it is a different form of the same verb, but mohu is more formal, may sound bookish in spoken (except very formal) conversation.
Děkuji! mohu is just easier to remember... but I'll keep this in mind when\if I have a chance to use Czech in real life
Why is 'I may not drink beer' wrong?
This is nesmím.
But here https://forum.duolingo.com/comment/27744981 you argued that moct and smět are interchangeable (as I understood, at least in colloquial speech). So why are they not interchangeable in this case?
"I am not allowed" should be a possible solution a.f.a.i.k.
I was clicking on words instead of typing the sentence, and I almost created this: "I cannot drink beer because I am dead." :D Sounds like a better excuse.