"Von to vokno nevotevřel."

Translation: He didn't open the window.

[JanisaChatte]

What's the reason for adding "v" at the beginning of words beginning with a vowel ?

[VladaFu]

Btw, some ancient (much older than "vokno" and similar) v's like this are now standard. "Vejce" (Old Czech "vajce") used to be *aje (cf. German "Ei") and some Slavic languages added "v" and some "j" at the beginning (jajco).

[endless_sleeper]

I don't want to get into any linguistic details (those can be found here https://www.czechency.org/slovnik/PROTEZE or in scientific papers (just google it)), but basically it's kinda easier to pronounce.

Btw, the very same phenomenon happens in Ukrainian too - він, відкривати, вікно.

[JBHayven]

This is also noted in very rural Polish (or at least the dialect of the mountaineers' from Tatry, whose unique trait is also initial stress): łon, łodkrywać, łokno (remember that "ł" is read as /w/, and this pronunciation originated in spoken dialects).

[VladaFu]

Also in both Sorbian languages. Even in the standard form.

[Alejandro1917]

The same even to a greater extent in Belarussian