We are not in Bergen then! :D
Hvorfor er det "å regne" og ikke bare "regner"?
Because 'sluttet' is past tense, and you can't have two different tenses in the same sentence. When the previous verb describes the next you should use the infinitive in Norwegian, while English sometimes uses the gerund form.
And how to say that someone stoped something to do other thing. Example: I stopped to smoke vs I stopped smoking .?
I stopped to smoke = Jeg stanset for å røyke/ta en røyk.
I stopped smoking = Jeg (har) sluttet å røyke. (If you omit "har" it sounds more like a temporary thing, while including "har" implies a permanent end.)
I stopped smoking = I quit
I stopped to smoke = I ended an action in order to have a cigarette