Translation:It is coffee
Why sometimes are subject pronouns omitted, but sometimes they're present?
If you want emphasis you include it
I wrote "He has coffee." Why is that wrong?
Είναι means "he/she/it is" not has
Why is coffee sometimes rendered as καφές, and sometimes as καφε?
Είναι καφές is plural, it should be Είναι καφέ
Καφές is singular, καφέδες is plural.