"He eats a sandwich."
Translation:Αυτός τρώει ένα σάντουιτς.
The word σάντουιτς has neuter gender -- that is why you need the neuter form ένα.
You would use μία before a feminine noun, e.g. Αυτός τρώει μία φράουλα "He eats a strawberry".
And before a masculine noun, it is έναν when, as here, it is the direct object, e.g. Αυτός τρώει έναν κουραμπιέ "He eats a qurabiya (kind of cookie)".
I think it's because "ena" is used for masculine (or genderless like with "to" )nouns and "mia" for feminine ones . Not entirely sure though.
The verb τρώω is used when we are eating something for example εγω τρώω ψωμί (I am eating bread) and the verb φάω is used when we are going to eat something for example Εγώ πάω να φάω ψωμί (I am going to eat bread)
PS this literally how it is translated belive me I am from Greece