"I do not know whether those windows are closed."

Translation: Nevím, jestli jsou ta okna zavřená.

[filigree2]

Is the use of neznám in this context incorrect? "Neznám jestli ta okna jsou zavřená."

[endless_sleeper]

It's incorrect.

[Kad759907]

"nevím, jsou-li ta okna zavřená" is an accepted answer.
Can anyone explain this 'jsou-li' to me? It looks totally unfamiliar...

[VladaFu]

It hasn't been taught yet, don't worry. "-li" is a conjunction with the meaning of "if" in the form of an enclitic.

[Kad759907]

ok, díky :)

[Avtokrat]

Why is wrong Já nevím, zda jsou tamty okna zavřená.

[endless_sleeper]

tamtA ^ 'Okno' is a neuter.