Does this mean the sixth floor counting from 1 or 0? As in, if the house has one staircase is the upstairs the first floor or the second floor in Swedish?
We use both systems and you can never be sure until you've seen what a specific building looks like. The elevator can show either B for street level and 1 for the next level, or B (or 1) for street level and 2 for the next. People of course refer to the floor the same way the elevator in that specific building does.
How do you know when you need to put a ¨det¨ or ¨den¨before the adjective-noun combination and when you don't? Some of these number sentences have them and some of them don't (and I keep getting them wrong, when they seem like exactly the same construction to me).
Most adjectives (and similar word classes) will need it. But some can't have it (samma, hela etc). Some others can have it, but don't need to. It's one of those irregular concepts one will have to learn. Keep practicing and you'll learn it soon enough.
Sex-> Sjätte Sjätte borjar med sj,lik sju, och man säger det nastan lik Sette på italiensk,som männer Sju. Det är konstigt.