Because French comparatives are usually built with plus, not with an ending like English -er.
Compare:
So plus calme is the normal way to say quieter.
Because calme agrees with appartement, which is masculine singular. Also, calme is one of those adjectives whose masculine singular and feminine singular forms look the same.
Examples:
You only see a visible change in the plural: calmes.
Yes. French ce / cet / cette / ces can mean this, that, these, or those, depending on context.
If French wants to be more explicit, it can add:
Examples:
So on its own, cet appartement can sometimes be understood as either this apartment or that apartment, depending on the situation.
In plus calme, the final s of plus is often not pronounced, so many speakers say something like plu calme.
A useful thing to remember:
Because pronunciation of plus varies somewhat by context and speaker, it is a good word to pay attention to when listening to native French.