"I do not know whether those windows are closed."
Translation:Nevím, jestli jsou ta okna zavřená.
Is the use of neznám in this context incorrect? "Neznám jestli ta okna jsou zavřená."
This translation is wrong. Nevim, jestli jsou ta okna zavrena. It makes no sense. You'd have to write - Nevim, jestli jsou okna zavrene. Again, I don't know if this is the Moravian way to translate it into Czech. [I'm Czech, but I haven't really had much practice in writing. I can speak it almost fluently though]
"nevím, jsou-li ta okna zavřená" is an accepted answer. Can anyone explain this 'jsou-li' to me? It looks totally unfamiliar...
It hasn't been taught yet, don't worry. "-li" is a conjunction with the meaning of "if" in the form of an enclitic.