"Twelve months is a year."

Translation: Двенадцать месяцев — это год.

[aegray]

The correct solution is "двенадцать месяцев - это год " why is это required here?

[deactivated user]

Because it sounds very unnatural without it. Please see my answer here: https://www.duolingo.com/comment/11559427

[aegray]

Your response makes sense, and I understand in the case of a chicken-bird (or some weird combination) - in this case is it required because it could be read as something like "a 12-month-year"? Is it only correct in cases when you could confuse the meaning? When would it be incorrect to use это in A - это B?

[deactivated user]

No, it's actually required in most sentences of the type «X is Y» where both X and Y are nouns, even when it doesn’t actually lead to confusion.

The sentence «Двенадцать месяцев — год» theoretically could be misread as «Двенадцать — месяцев год», whatever it means ('Twelve is the month's year'?), but это is more of a grammar thing.

I've gotten a ton of problems wrong when I include это, and it feels like I'm slightly guessing about its presence

Hm... Maybe you were using it before adjectives? :? Not sure. Unfortunately, I don't know where to find exact rules for using «это».