This is sooo interesting to read!! As a native speaker I don't really think about it but it's really fun to have an outsider's view of what is natural to us. And i've been learning japanese for two years now so I know your pain (I learned English by osmosis, I never had to really study). I know your're specifying portuguese because that's what your're learning, but I believe that most of those point's apply to other romance languages as well. If you have any specific doubt I'd be happy to help.