I know DuoLingo wants to distinguish between DEFINITE and INDEFINITE, but any native speakers... Could you think of examples wherein "a window" would be translated as "ek ढारवज़ा"... ?
The absence of एक doesn't tell you anything about definiteness and you would infer it from context. However in cases where both interpretations are valid, absence of an article is usually taken to indicate definiteness.
Consider a sentence like "A window in my room is big" You can't translate it as " मेरे कमरे की खिड़की बड़ी है" because that sentence seems to suggest that there is just one window in your room. So, the एक is necessary in this case. मेरे कमरे की एक खिड़की बड़ी है।