"We often go to museums."
Translation:Vi går ofta på museum.
Vi går ofta på museer is an accepted answer too. But the Swedish sentence Vi går ofta på museum is the main sentence: the Swedish sentences are always written first. In this Swedish sentence, gå på museum is used without an article in a way that doesn't really have a counterpart in English. (gå på ett museum would be 'go to a museum' and gå på museet is 'go to the museum'). gå på museum is a more general expression. Its meaning is something like "go museum-going", if you see what I mean. Because of this, using the plural is the best English translation.
There are some similar expressions where we use an indefinite noun without an article: gå på bio 'go to the movies' works exactly the same. köpa bil 'buy a car' is another example. It's quite possible to köpa en bil, and we often say that too, but köpa bil has a more general meaning, where köpa and bil form a sort of unit.
I wrote vi ofta gå på museum and it says the correct answer should have been vi brukar gå på museum. But then in the comments section it says vi går ofta på museum. Can somebody explain the difference between the two correct answers to me and why my option is wrong? Because I'm a bit lost right now.
There are two mistakes in your suggestion:
1. The present tense of "gå" is "går", so it has to be "vi går".
2. The word order in your sentence is not correct. The normal translation is "Vi går ofta på museum", but "Vi går på museum ofta" works as well.
ofta = often
vi brukar gå = we usually go
It's a time adverbial and it isn't wrong per se to put it first in the sentence. It's just that doing that changes the meaning of the sentence so much that it's no longer likely as a translation of this English sentence. The meaning becomes a lot more specific, more like 'A lot of the times, we go to museums' or 'What we do a lot is that we go to museums'.