Something that I wanted to discuss here is maybe familiar to you, but I would still like to make it clear in case it confuses some of you.

In previous topics, when I wrote some of the examples and also in every day speech or when you read something in German, you can see forms in/ins, an/am, in/im etc. It might confuse you - how do we even determine which one to use - why do we change preposition?

We don't actually change preposition - we just combine the article that goes after it with that preposition, so it is easier to pronounce it that way. That's also the reason why we don't do it for all the articles, sometimes it would just be impossible to say it, which would make it lose its point.

1. im - in dem
2. ins - in das

There is no such a thing as ir - in der, it wouldn't sound normal to say it that way

1. am - an dem
2. zur - zu der
3. zum - zu dem

This means that we can say all of these structures both ways and still be completely correct:

Ich gehe ins Kino. / Ich gehe in das Kino.

Ich gehe zur Schule. / Ich gehe zu der Schule.

Maybe in the beginning, you will find it easier to use longer version because you have the time to think about: which preposition to use + what case goes with it, and not have to stop and think all of that plus how to connect them, but later it will be easier for you to say it this way and it will come naturally.