"This is not a boulevard, it is a street!"
Translation:Acesta nu este un bulevard, este o stradă!
This is tricky, because there are two clauses. Ignore the second one and it becomes obvious that "acesta" must agree with the only thing left to agree with:
"This is not a boulevard." - "Acesta nu este un bulevard."
However, if a pronoun were to show up in the second clause, it would indeed have to agree with "stradă", so you end up with two forms, even though they logically refer to "the same thing":
"This is not a boulevard, this is a street!" - "Acesta nu este un bulevard, aceasta este o stradă! "
Thank you kindly for your explanation but the question remains... In the first sentence we are talking about a 'street' that is NOT a boulevard... If you would, "aceasta" is (should be) an abstraction (cale, ruta, etc) of both street and boulevard... Also, if I look at a street, then I can say "Aceasta nu este un bulevard ci o strada..." What am I missing? Regards.