Quote Originally Posted by possopo
there are two cases i have a real problem with, the first being genitive when used in the negative form, the second being instrumental used when, ahem, well, i don't really know how to explain it.

why are you saying:
ostavit' okno otkrytym and not simply otkryto.
on stal ingenerom instead of ingener.

an explanation of this strange phenomenon would be highly appreciated
The instrumental case is used in these sentences because they denote a change of state (from closed to open, and from [something] to Engineer).

As with every change of state that a occurs, the instrumental case is used. For example: Ya xochu byt vrachom - i want to be(come) a doctor.

Hope that helped.

(P.S. Correct me on this one, guys)