A couple of questions, mainly なければならないんです　Vs　なければなりません
 

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 :vsign:

There is a subtle but important difference between ~ないんです and ~ません.

The latter is the common "textbook" form without any connotations. Native speakers might use it in writing, but not use it too often in speaking because it, to us, clearly sounds textbook. .

The former, which you will probably hear more often than the latter, is used when telling a fact to a person who the speaker is sure didn't know about it. What he tells will be "news" to the listener.

This is actually pretty advanced stuff. I say this because it's easy to explain the difference and many second-year students would learn it but it usually takes Japanese learners more than a few years before they can use these properly.
________

You call it negative. Is it? Look harder.

It's double-negative, meaning it's the same thing as affirmative.

買わなければなりません

It's kind of like saying in English "You can't go without ~~~" instead of "Go with ~~~."
In either language, the speaker isn't so actively aware if what we are saying is affirmative or negative.

Just to add a little bit more to his answer. のです/んです can be seen as the English verb "happen" or "you know". Like, "you know, we have to buy tickets before enetering the theater" or something of that effect. Many times unfortunately it doesn't have a real translation.

You will learn that some things in Japanese are better to be learnt as per what their role in the phrase is, instead of what they can be translated into. A very good example can be the verb 置く(おく)used as auxiliary after the て form of verbs. I think in many languages it is always translated as a plain future.