Translation:Ronaldinho thought he would arrive late.
It's not in the course itself, you'll have to rely on external ressources.
You'll find out about diminutive and augmentative forms in Portuguese which (to me, at least) is quite faszinating.
Also, check out the stories. They extend the vocabulary in the tree significantly, the Duolingo guys have done a great job there. Vale a pena, seríssimo. <3
You could guess that it could translate as a cute form of Ronald. I had a friend whose child was "Vasco" but everyone called him "Vasquinho" because it was cute, but this would not be expected to continue past childhood. So perhaps "Ronnie" is appropriate in some way. But I don't think it is appropriate to translate a name just because you can. If my name was Alberto, I would expect people to call me Alberto wherever I am in the world, not deciding themselves to call me "Albert". Similarly I expect that Portuguese speakers should call Apple iPods, "Apple iPods" and not "iPods de Maçã". Do you agree?
That may be the case typically, but not always. When being referred to in Portuguese I don't mind being referred to by the portugueses translation of my name (particularly if the person with whom I'm speaking doesn't speak English). So, in your example, if my name is "Albert", while speaking in Portuguese referring to me as «Alberto» is fine with me (though we might establish that when we first me or start speaking).
I had the same question about the use of -inho, and found this helpful. http://www.learn-portuguese-with-rafa.com/Learn_Portuguese_with_Rafa_Newsletter-diminutives-in-portuguese.html