I came across this sentence:
映画館に入る前に切符を買わなければなりません
（We)need to buy a ticket before （we） enter the movie theatre.
A few questions here.
Firstly, what is the difference between using ~ないんです and ~ません in requirement conjugations? As far as I'm aware, they're both polite, right? I ask mainly because Pimsleur II's audio uses the ならないんです for 待たなければならないんですand the above sentence is なりません.
Secondly, why is the conjugation negative? Basically, the sentence is we DO (postive) need a ticket, as apposed to we DON'T (negative)? Or does 日本語 see a requirement as a negative thing in general (more like an obstacle) and as such if a requirement exists, it's a bad thing so we use ません or ない？
Cheers in advance
