"Jestem w pracy."

Translation: I am at work.

[Darth_Elven]

Dlaczego w pracy, a nie w prace?

[Jellei]

Well, that's just how it is, I'm afraid ;)

[Estelars1]

Para los hablantes de español w=en
Estoy en el trabajo.

[GeraldPres]

Why is 'I am in work' wrong?

[fastas1]

See my comments on this below.... I, and every other native speaker in all parts of the UK i have lived in or visited have used either at or in for being "present at one's place of work". Yes, "in work" can also mean other things depending on context, but so can "at work".. However because the moderators 2 native speakers say no, then it isnt added. So yes it should be accepted but it isn't, cest la vie.... :)

[TwinTip]

So, when I say I am somewhere, I use the genitive case?

[immery]

In this sentence it is locative. (and if something happens or is w something you use locative case.)