"Kobieta nie je posiłku."

Translation: The woman is not eating the meal.

[JamesTWils]

Is there any way to tell when the genitive will be -a and when it will be -u?

[immery]

@vytah answered this question today, but short version is - no,
https://www.duolingo.com/comment/13088911

[JamesTWils]

Thanks for the link. I will just memorize then. I thought there was a possibility that -u was just for partitive genitives, but that illusion is now dispelled.

[viki98431]

Can I say "kobieta nie je posiłak?

[Jellei]

"posiłak" is not a word, but I will assume that this was just a typo. But no, "Kobieta nie je posiłek" is not a correct sentence.

So basically, "jeść" (to eat) takes Accusative, which here is "posiłek". But it's negated. When a sentence using Accusative is negated, it takes Genitive instead. "posiłek" turns into "posiłku".

That is the only situation when negating the sentence changes the case. Other cases stay the same.

[cszerszen]

Wiki shows genitive as posiłka. Is this incorrect?

[alik1989]

It is indeed incorrect. Unfortunately I have no idea how to edit a declension table. The Polish page is correct, though.

[Vengir]

I've just corrected it

[Walkinthedog]

Could it also mean snack?

[alik1989]

Not really. That's too much of an interpretation.

Snack = przekąska

[IlliaSokolovskyi]

'the woman doesn't eat meal'
is it correct?

[Jellei]

It needs an article before "meal".