Translation:I have decided to tell him that I love him.
I find it funny that an earlier sentence in this lesson was, "Voglio di dirgli che lo amo."
That sounds strange to me. As far as I learned, after 'volere' verb goes directly, without a proposition. So, in my opinion, this sentence should be: "Gli voglio dire che lo amo"
The previous question was 'non dirli la risposta' for 'don't tell him the answer', but now it's 'dirgli'. Is there a rule here to help the confused?
dire a lui Ho deciso di dire a lui che lo amo. = Ho deciso di dirgli che lo amo.
"lui" is the personal pronoun which is generally the subject of a sentence. him, her etc. are the object pronouns (in Italian you have an direct object pronoun and an indirect object pronoun. For both exist an accented and an unaccented type.
I answered - I have decided to tell him that I love it, and it was accepted.