"The man eats an apple."
Translation:L'uomo mangia una mela.
Don't think of masculine, feminine as descriptive they are just ways of showing how words are declined. Sometimes as you see we call an apple in Italian feminine. In German a girl is neutral, a jacket is feminine, and in Greek a girl is neutral but a chair is feminine. So, it's better to just think of nouns belonging to different groups. And most of all learn each gender when you learn the noun...not just mela but LA mela it will make life much easier later on. Can't count the hearts I've lost in German because I didn't learn the genders early on.
Una is for nouns that are feminine like mela, un is for nouns that are masculine like ragazzo, look at the last letter, for the most part nouns that end in "a" are feminine, nouns that end in "o" are masculine. Just from observation, I believe that when a masculine noun becomes plural the last letter will change to an "i" , for female nouns it will change to an "e".
The infinitive is "mangiare", and regular verbs ending in "-are" have "a" in the end for the 3rd person form, but the base is "mangi-". It's a pronunciation thing: if the letter "g" is followed by "e" or "i", it is read like in the English "gem", if by "o", "a", or "u" then like in "gum".