Translation:Αυτό το μήλο.
Το μήλο is neutral, as the article το indicates. Therefore, it can only be matched with the neutral pronoun αυτό. Use αυτός for masculine words only and αυτή for the feminine ones. e.g. Αυτός ο τοίχος - This wall, Αυτή η καρέκλα - This chair.
So, in Greek there is no separate word for "this", you have to say "he/she/it the + noun"?
Or the other way around: in Greek there is no separate word for "he", you have to say "that (one)" instead.
η and υ are pronounced identically in modern Greek, so I'm not quite sure what you're getting at. They both have the sound of the English letter name "E", as in "me".
Is there a trick to knowing when you use certain letters? I put "αυτο το μιλω" so I'm not sure when to use Η,η/Ι,ι or Ο,ο/Ω,ω
Concerning our example, whenever a neuter word ends in an "o" sound, it's spelled with an "ο". Whenever a verb ends in an "o" sound, it's an omega (ω) ;)
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.
Could someone please tell me what the correct pronunciation of "υ" as in "αυτό" is because I hear it as an "f" as in "afto" here but other sources say differently.