How do I know when to use mangia, mangi, or mangio?
I'm not learning Italian, but I know a bit of French, so I'm guessing that those are different conjugations of the verb meaning 'to eat'. They are different because in many languages, there are different conjugations, or forms of the verb, depending on who you are talking about. For example, in Spanish, como means 'I eat', comes is 'you eat', and come is '(he, she, it) eats'. This is different than in English, where we would either just use 'eat' or 'eats'. So mangia, mangi, and mangio are just different forms of the same verb, and to know which one to use, you have to know which one goes with each pronoun (I, you, he, she, it). I hope this makes sense and I hope I helped you.
Lei, Lui mangia; tu mangi; io mangio. They are different conjugations of 'mangiare' (to eat). In many languages, the infinitive ending (in this case, '-are') is removed and a different ending is added depending on the speaker. This way, subjects can also be left out, and the sentence will still be understood. Io mangio - I eat Mangio - I eat Io ('I') is not included in the second sentence, but one can still tell you are referring to yourself based on the io ending -o.
Mangiare is a regul. verb ending on -ARE. So just follow the conjugation rule for such verbs. And also they are verbs ending on -IRE and -ERE.
Mangi-ARE: (io/I) MangiO (tu/you) MangI (lui, lei, Lei/he, she, you-formal) MangiA
(noi/we) MangIAMO (voi/you plural) MangIATE (loro/they) MangIANO
Mangio una mela/ I eat an apple Mangi una mela/You eat an apple Mangia una mela/He, she eats an apple (plus formal) Mangiamo una mela/We eat an apple Mangiate una mela/You eat an apple (plural) Mangiano una mela/They eat an apple
Another thing to add: "Mangia" is imperative for the second singular person as well. :)