"You are eating this sandwich."
Translation:Εσύ τρως αυτό το σάντουιτς.
Αυτή is used only for feminine nouns. Sandwich (σάντουιτς) is considered neuter. In other words:
Αυτός (masc.), αυτή (fem.), αυτό (neut.).
And no, you definitely can't omit the article το in this case. It's a basic greek sentence structure when you're talking about one specific thing. You could only omit the article in really general cases, without the pronoun αυτός-ή-ό, like
-Εγώ πίνω γάλα - I drink milk (in general. You wouldn't say "πίνω αυτό το γάλα", unless you were referring to one specific bottle of milk.)
I hope I helped you a bit. ^.^
Because είσαι means you are in the sense of being. It's like saying "You are are eating/eat a sandwich" in English. It doesn't make sense. :p
Εσύ is the pronoun you should use for you, and τρως for eat/eating. Present Simple and Present Continuous are expressed by the same tense in Greek, Ενεστώτας.