why is "we often go to theater" not accepted ? how can the translation: "We often go to the theater." be the correct solution and there is no definite form in the sentence.?
I don’t think ”we go to theater” is idiomatic English; English uses a definite article whereas Swedish doesn’t.
Actually the way I learned from Oxford Grammar is that if you go to see a play, you could say "go to theater", while "go to the theater" implies going to the building, but not necessarily for the purpose of seeing a play. Similarly go to (the) school, being in (the) hospital, without the definite article implies going there for their primary purpose, with the article emphasises going to the building or location, may be for some other purpose. I found that this is not a hard rule, especially in the US, but there is a tendency of using one over the other depending on the purpode. But either way, "go to theatre" is idiomatic at least in some English variations (Google search provides many examples).