Translation:Αυτό το μήλο.
In general, it's something you have to learn, much like "ee" versus "ea" in English: why is "meet" (encounter, come across) spelled with "ee" while "meat" (edible flesh of an animal) with "ea"?
There are historical reasons (they used to be pronounced differently) and the same is true with Greek -- but in both cases, the pronunciations have merged and so in the modern language you just have to memorise it.
As G.Georgopoulos has pointed out, you can usually tell the correct spelling with endings, but in the stem of a word, it's pure memorisation.
Use this link:
https://forum.duolingo.com/comment/22040507 It show you:
-how to access the Greek keyboard
-show you the full Greek alphabet
-how to find the letters on the keyboard
-how the letters are pronounced
-how to add accents
-and much more.
And this is another good link https://forum.duolingo.com/comment/30960700
Also use these:
TIPS TO HELP YOU LEARN + HOW TO REPORT A PROBLEM
And check out the Greek Forum here with more links.