It's really hard, actually. Ask five native speakers and you'll get five different responses. The same goes for searches, you'll get a range of responses.

"Must" can have an external obligation, eg. "Employees must wash their hands"; "All students must study English in my country". In the second sentence, you could use "have to", but the meaning would be less authoritarian. In the first sentence, "have to" looks unnatural because it's clearly a rule imposed by management.

There's also a lot of overlap. Honestly, I think this is an aspect of language you should just try to "feel" rather than study. The more you read, the more you listen, the more you will understand the difference instinctively.

If you'd like to continue this discussion, I suggest you post some example sentences that have confused you, and we can analyse them for you.

You can say both, but in different contexts. The native guys can explain it better, but as Alan said, you have to 'feel' when to use one of them. For example:

'The doctor said I have to drink a lot of water to keep myself hydrated.'

'The doctor said I must drink a lot of water if I want to be able to treat this recurring malaise.'

I Got it. 

Thank you guys.