"He eats a potato."
Translation:Αυτός τρώει μία πατάτα.
Because πατάτα is feminine gender, like pretty much all nouns that end in alpha (except those in mu alpha -μα, which are usually neuter).
ένα is neuter.
In older spelling, it could also be the form for the masculine accusative έναν before certain consonants, but the current spelling rules mandate that it always stays έναν.
No, μια/μία means "one" (or sometimes "a, an").
You may be thinking of Spanish or Italian where "mia" means "my".
y u no rite hole sentence?
Αυτός is masculine, αυτό is neuter - at least in the nominative case, such as we need here for the subject of the sentence.