The verb "lembrar" does not require direct objetcs like in english as "I remember you". Instead, it usually requires an indirect objetc, and that is why the preposition "de" is used like in "Eu lembro de você". That would be equivalent to say (but wrong!), in english, "I remember of you" .
I think it's correct, so what were you expecting? I had a quick look at "lembrar" in my vocab list and there were only a couple of cases where it was not followed by "de" (or a "de" based contraction).
Davu, Thanks for the response. What is what every language learning wants: a clear rule. I think what you are saying is add "de" and 80% of the time it is right.
Not really, I was asking what led you to think that "de" was wrong here. You must have seen some other use which clashes with this one. If you could say what that was, perhaps a native speaker could explain the reason that the other case is different.
When referring to my vocab list I should have said that all the sentences which have a similar context to this one use "de". There are other uses for "lembrar" though as shown in this dictionary entry: http://dictionary.reverso.net/portuguese-english/lembrar and you'll see that "de" is optional or missing in some.
Davu, Cool thanks for the resource. Regarding why I thought this could be wrong, the truth is I didn't know. I am trying to clear up my own confusion. So far I have been muddling through. Since I have found mistakes with the translation of the cards I want to know am I wrong or have I discovered a bug. Based on the source you pointed me to I think I have a rule here. In English we talk about recalling a fact or memory two ways. We use "remember" to describe recall within one's self without external prompting, e.g. "I remember you" and this is "lembrar de" in one of its forms. However, if we are talking about recall being stimulated outside the self then we in English use "remind" which would be for example "ele lembra meu irmão".
The Duolingo model is to learn by making mistakes and it is a very blunt tool. It can't really be used on its own in my opinion. There are many other useful resources though. Certainly dictionaries can help (although even they can deceive sometimes) see:
Trying to find the correct preposition for a verb is always a problem and one thing I mention there (http://linguee.com) is really useful in this regard (as long as you are aware that translators can make mistakes sometimes and don't take an isolated case as definitive).
See the examples:
Eu lembro DE você.
Eu lembro você DE alguma coisa.
I remember you.
I remember something to you.