"I eat a banana."
Translation:Je mange une banane.
I don't know about answer nb1, but "une crepe" is singular "des crepes" is plural.
The same as between "eat" and "eats": conjugation.
French conjugations are more extensive and the endings change with the grammatical person:
- je mange, tu manges, il/elle/on mange, nous mangeons, vous mangez, ils/elles mangent
"une" is not linked with "I" but with "banane" which is a feminine noun.
The English "a/an" has no feminine nor plural forms, but in French, "un" is masculine, "une" is feminine and "des" is the plural form of "un" or "une".
j' is used before a verb starting with a vowel sound: j'aime, so as to help pronunciation.
if the verb starts with a consonant, there is no need to elide "je": je mange.
It's just grammar. You just have to learn whether a noun is masculine or feminine by looking at the definite article.
For example, "the apple" is "la pomme". That means that "apple" has the grammatical gender of feminine. "La" is the feminine singular form of "the".
There isn't a lot to say about masculine and feminine other than you just have to learn which noun is masculine or feminine.