"She tells him she loves him."
Translation:Lei gli dice che lo ama.
Gli = a lui
Lo = lui (as object)
You can see this sentence using pronouns instead of particles.
Lei dice a lui che ama lui.
(It's not natural, but to explain to you the difference it's the best way)
You should find listed both particles in the FAQ at #11 http://duolingo.com/#/comment/233855
In English, we can say "She tells him", which doesn't require the preposition "to". Or we can say "She says to him", which does require the "to". It seems to be similar here. Lo = him, Gli = to him. So Dire must require the preposition form and Amare doesn't. Does that sound logical to anyone else?
I think the key here is that there is no Italian analog to "tell" in English. You must always "say" something "to someone" rather than simply "tell someone." Notice that the verb is "dire" -- to say. You are on the right track for sure -- "dire" requires the preposition, as does "to say."
- Lo dico -- I say it.
- Lo gli dico -- I say it to him.
- Gli dico -- I tell him.
Hope that helps, because it's helping me to explain...
Yes we can say "She tells him". But "him" is nonetheless an indirect object (IO) the DO is what she is telling. If you put in both objects you get "she tells X to Y" so X is DO and Y is IO. It is a quirk of English that when the DO is absent with "tells" we don't use "to" with the IO.
I wish Duolingo would be more consistent with its inclusion of "that". In this case it doesn't include it in the English but requires the Italian equivalent, Che, in the answer. In other exercises it uses it in the English but then doesn't need it in the Italian. It's really confusing