Questions & Answers about У входа в театр мы ждём друзей.
У + Genitive commonly means by/at/near something (a location next to a reference point).
So у входа literally means by the entrance / at the entrance (not “inside” the entrance).
After у, Russian uses the genitive case.
вход (nominative) → входа (genitive singular).
So: у + Gen. → у входа.
Because вход в театр is a fixed-type construction meaning “an entrance into/to the theatre” (direction “into”), so it uses в + accusative:
- театр (accusative singular) = театр (same form as nominative for this noun)
If you meant location “in the theatre,” you’d use в театре (prepositional):
- в театре = “in the theatre”
But here it’s “the entrance (that leads) into the theatre,” not “the entrance located in the theatre.”