"We never used to walk on that street."
Translation:Nós nunca costumamos caminhar naquela rua.
This sentence is confusing, and I'm not sure I understand what the Portuguese really means in English. This is made particularly difficult because it includes a negative word, "never". I really think this sentence should be moved into a section on imperfect verb forms, but maybe I'm wrong.
Paulenrique, I also think you might have meant "We never used to", rather than "We are never used to." (If so, then the Portuguese makes more sense, to me at least.)
"We never used to" and "We are never used to" don't mean the same thing in English.
"We never used to" is a type of habitual past form (I may be using the wrong grammatical term, but it means we never did it on a regular basis in the past, but maybe we do now). It's often translated by an imperfect verb form into other languages (for those that have it), similar to, "We were never [doing whatever]", although to say "We were never walking on that street" is a really weird sentence in English (Maybe in response to someone accusing you of always walking on that street, when what you mean is a more emphatic form of "We were NOT walking on that street!")
To be used to something means that it's something you've seen before, maybe done before, and that you accept as normal. "We are never used to" sounds weird in English. You would more likely say, "We were never used to," or even "We never got used to..."
At this point, I'm confusing myself and going on for too long, but this page might help: http://tinyurl.com/op7za7e
I had it as multiple choice and imperfect was not offered. This same sentence in Portuguese, with the preterite, was marked wrong when I translated it as "used to walk." Duo's approved translation that time was "we use to walk" (which I don't believe is ever correct in English under any circumstances).
Now, Duo says "used" is correct.
For a discussion (including a reference from NOAD) which confirms that "we use to" is not correct English, see: http://english.stackexchange.com/questions/30035/i-use-to-or-i-used-to