Έχω μια μεγάλη αποσκευή να παραδώσω πριν περάσω από τον έλεγχο.

It breaks down like this:

Έχω = I have
μια μεγάλη αποσκευή = a large piece of luggage / a large bag
να παραδώσω = to hand in / to check in
πριν περάσω από τον έλεγχο = before I go through the checkpoint / security check

A very literal structure is:

I have a large piece of luggage to hand in before I pass through the check.

So the key pattern is:

έχω + noun + να + verb
= I have + noun + to + verb

Because αποσκευή is a feminine singular noun, so the article and adjective must match it.

• μια = a / one, feminine singular
• μεγάλη = big / large, feminine singular
• αποσκευή = piece of luggage, feminine singular

Greek adjectives agree with the noun in:

• gender
• number
• case

So if the noun is feminine singular accusative, the adjective must match that form.