"You must not commit more crimes!"
Translation:Du får inte begå fler brott!
I was curious about "fler", "flera", "mer" and "mera", found this: http://www.thelocal.se/blogs/theswedishteacher/2011/01/24/mer-mera-fler-or-flera/
The word "måste" is like "must", something that you have to do. So "måste inte" means that you don't have to do it.
As an auxiliary verb, får means that you "get to" do something, which in this sense means that you have permission to do it. So "får inte" means that you don't have permission to do something.
Even thought the literal translation of individual words would suggest using "must", the meaning of the English sentence requires that you use får.
få in the sense of "may" is always a modal. It's just that it can mean "receive" as well, in which case it's a different, non-modal verb.
The second sentence works just the same as in English: "We all commit crimes sometimes" - just the v2 rule dictating that Swedish move the verb one step, so that it takes the second position.
This sentence may have been a mistake, since we're aiming for US English specifically. In UK English, "you must not" and "you may not" are the same thing here. If you want to be strict about "you must not", it would be du måste inte, and "you may not" would be du får inte.
"Du skall inte begå fler brott!" borde väl ändå fungera i denna mening tycker jag? Här satt jag och avnjöt denna aftonstund när jag bemöts den här J*vla duolingo fågeln som säger att min välkonstruerade mening är fel. Skäms säger jag. Och inte hjälper det att jag streamade detta för att bevisa min talang i det svenska språket. Usch jag får faktiskt sura uppstötningar.
The thing that annoyes me, as a swede, is that "Du får inte begå fler brott" more accurately would be translated into "You are not allowed to commit more crimes". A more fitting translation for "you must not" I think would be "du borde inte/du måste inte/du ska inte".
(also, I am only doing the exercises because I wanted to know what they are like :P )