"You have a sandwich."
Translation:Εσύ έχεις ένα σάντουιτς.
The rules for needing "ενα" for "a" are not explained. I thought it was optional so have not included it here.
The indefinite article can be ommited only when the content of the sentence is general, and there is no confusion about the number of the object. In this case, I guess someone could have more than one sandwich, although I do think it could be ommited. ^.^