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Questions & Answers about Опера в театре.
Since we are learning about destinations, why is it в театре and not в театр?
This sentence describes where something is currently located, not where someone is going. For location (answering "where?"), we use the Prepositional case: в театре. If you wanted to say "I am going to the theatre" (destination), you would use the Accusative case you are learning now: в театр.
Where is the word for "is" in this sentence?
In Russian, the present tense of the verb "to be" (am, is, are) is almost always left out. You simply put the subject (опера) right next to its location (в театре) and the "is" is understood!
The English translation says "at the theatre". Why do we use в instead of на?
English often uses "at" for locations, but Russian usually splits this into в (inside an enclosed space or building) and на (on an open surface or at an event). Because a theatre is a physical building you go inside, Russian uses в.