What is the rule for changing the preposition "в" into "во"? I cannot think about any way to explain it.
First of all, semantically they seem to be perfectly identical:
(дежурить) во вторник, в среду.
Both forms can be used with the same word:
(увидеть) во сне, (провалиться) в сон.
Even phonetic conditions may be the same:
(пройти) во двор, (стучаться) в дверь
Any ideas?
вdoesn't always becomeвоeven when the following word starts withвitself, for example в вечность or в ворота. – Aleks G Sep 27 '12 at 15:03