i = plural form of il, le = plural form of la

il panino -> i panini --- la farfalla -> le farfalle

Why can't you say "I has panini"?

Because it's io ho panini

You can, you just don't have to. You could think of it like this: conjugations in Italian actually change depending on who is doing what. In English, the only distinction is he/she/it has the rest are have. So you get a lot of details from "ho i panini".

tramezzino usually is a sandwich made with bread for toast cutted in the shape of triangle, panino is when it is made with normal bread

me too!