"The boy is eating an orange."
Translation:Le garçon mange une orange.
In either language, you don't use 2 conjugated verbs in a row.
"is" is an auxiliary and it is conjugated and "eating" is the present participle/gerund form of "to eat", so it is not conjugated.
"is eating" is a specific verbal form called "continuous present" and it does not exist in French.
"is eating" therefore translates to "mange", in present, and "eats" also translates to "mange". Context will tell the difference.
Because the feminine indefinite article "une" ends with a consonant sound (N); thus, there is no vowel sound conflict with the next word starting with a vowel sound.
You learn each noun with its gender as you go.
orange = [une orange] - just keep the article as if it were a prefix.
You don't. In the same way as there is no reason for a ship to be "she" in English. It just is.