Why is "Окна" plural?
nom. sg.: окно́ (neuter)
nom. pl.: о́кна
gen. sg.: окна́
The stress matters.
The normal pattern for neuter nouns is: Change о to а (similarly е to я) and change the stressed syllable. Genitive singular is usually the same but without the stress shift.
Doors and corners, that's where they get you..
What is the difference between окна and окно?
"Окно" = a window, "окна" = windows.
Why does the pronounciation soun slike дверя or двере?
Another oeuvre of Russian world literature, I assume?