"We have a guest!"

Translation: Mamy gościa!

[Elias.M.A]

What case is "goscia"?
Accusative?

[Jellei]

Yes. Mamy kogo? co? (We have whom? what?) = Accusative.

[immery]

yes it is accusative

[ThorstenAl3]

Why isn't it "goscię"? Or does this only apply for real female and not words with an a at the end which are masculine?

[Jellei]

But this word doesn't end with -a. Well, this form (Accusative) does, but not the basic, Nominative one, which is "gość".

The masculine words ending with -a also get -ę ending in Accusative.

[ThorstenAl3]

Ah! Ta again.