Re: corect grammer
Everything depends on context.
You get a text message while you are waiting for the show to start. You text back: "I"m at the theater. I can't talk now."
It doesn't matter whether that he/she doesn't know which theater.
You are supposed to meet you friend at the theater. The show is about to start but she still has not shown up. You go in and take your seat. You get a text: Where are you??" You text back: "I'm in the theater. Come quickly before it starts!"
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.