"My best friend is a realist."
Translation:Ο καλύτερος μου φίλος είναι ρεαλιστής.
Because we don't need to use indefinite articles when there cannot be any confusion about quantity.
Your best friend is never going to be two realists, so we don't need to use it.
However - something like: θα ήθελα ένα μήλο
We definitely need to use it, as I could want one apple, or two apples, or three apples....
I really struggled with this at first. However, if you mentally change the 'a/an' to a 'two' in the English construction of the sentence, and pluralise the noun, that'll tell you whether you need the indefinite article in Greek or not. If you wouldn't ever put a 'two' there in English, don't use the indefinite article in Greek.
If a 'two' would work there in English, then give the Greek sentence an indefinite article.
Hope that helps!