Learn Hardcore Russian: Если в кафе слишком шумно, мы посидим в парке. - If it’s too noisy in the cafe, we’ll sit in the park for a while.

In Russian, a main clause and a subordinate clause are normally separated by a comma. Here you have a conditional subordinate clause introduced by если (Если в кафе слишком шумно, …) and then the main clause (мы посидим в парке), so a comma is required.

What grammatical “case” are в кафе and в парке in?

They use в + Prepositional (also called Locative/Prepositional) because they answer где? (where?):

в кафе = in the café
в парке = in the park

You can compare with motion (куда? where to?), which would usually be в + Accusative: в парк (to the park).

Why is кафе not changing its ending (like в кафе, not something like в кафее)?

кафе is an indeclinable noun (usually a loanword). It stays the same in all cases and numbers:

в кафе, из кафе, к кафе, etc.

You just learn it as “doesn’t decline.”

What part of speech is шумно here, and why isn’t it ?