Before Nico and Will started dating Nico realize he's falling in love and HARD. He talks to Piper about it and Piper says,"Just tell him!" "It's not that easy!" Nico grumbles, but he tries his best. Problem is, when Nico gets nervous he tends to speak Italian. So he runs up to Will in the infirmary and Will is like,"Hey Nic-" and Nico just shouts "TI AMO!" at him and then show travels away to hide and rethink his life. After calming down for a bit Nico realizes that of course, Solance doesn't know he loves him cause Nico confessed in ANOTHER LANGUAGE but, little did the Son of Hades know, Will taught himself that very same language in hopes of getting closer to Nico. Thus meaning that Will understands the phrase.
Credits to: An unknown person that I want to meet.