The Greek words you will want first, with English translations and part of speech, sorted into 22 topics. Free to read, no sign-up. Greek letters, with a romanization to sound them out. The full packs live in the app.
Watch Your Language puts Greek words like these in a widget on your iPhone Lock Screen and on your Apple Watch face. You see a handful of them every time you check the time, and the app schedules the repeats so the words you keep forgetting come back more often than the ones you already know.
You can add your own words too — from a photo, a page of notes or a spreadsheet.