This is a simplified explanation... Not all neuter gender get "et", some only receive "-t": e.g. ett piano - pianot ett suddgummi - suddgummit ... And not all plurals receive "-na": e.g. två hus - husen två äpplen - äpplena ... And some plurals even lose an -e before getting the ending "-na": e.g. två arbetare - arbetarna två stockholmare - stockholmarna ...
To a certain extent. There, the definite ending only occurs when the noun is functioning as the direct object of a verb - for example, the word "kız" in "kız su içer" could mean "a girl (drinks water)" or "the girl (drinks water)" since here it's a subject, and subjects don't receive an ending. But when "kız" becomes a direct object, it gets an ending -ı if it's definite: "ben kızı görürüm" 'I see the girl' versus "ben kız görürüm" 'I see a girl'.
By some metrics Portuguese is the most close to Romanian in the Romance languages family. Indeed, romanians can't understand spoken Portuguese (but understand more or less easily Italian, Spanish and some French), but I was amazed how much words and meaning I could recognize from a Portuguese WRITTEN text.
I found a decent site that explains this.
https://www.duolingo.com/courses - it's currently at 6.19k for Romanian, so you can't really tell the number anymore, but if you sign up when it's below 1k, it shows you exactly hom many students of a given language are there at the moment, though I have an impression that these numbers are not being updated live, so I'm probably #885 here, or at the very least I'm below one thousand, but who knows (and who cares after all =)
I remember being a student # thirty-something in one of the other courses, but I didn't really pay attention to this, although it's a nice feeling to be among the pioneers :)
Latin had no article. So Romanian "chose" a suffix to use it as one. Most Western Romance (if all) preferred to transform a indicative pronoun of Latin as an article. I guess it came under the influence of Germanic ones. Romanian suffixed it instead of a separate one, I guess. But I am not sure if it is the Latin suffix -us or -um that became a definite article or it is no article as it is in Latin or an indicative pronoun suffixed. I am not advanced enough to know all grammatical phenomena in Romanian.