Greetings all once more
Olaszinhok (# 17) is quite right. There are so many different aspects to learning (or using) a language other than one's native tongue: there is vocabulary, the grammar, pronunciation, idiom, accentuation, intonation, emphasis, and sometimes just the alphabet. And it also depends rather on what the student needs to learn it for: I became first acquainted with German (for example) through classical music, but had to force myself to learn to read it for academic purposes when I was a student. I acquired a passable competence (for those purposes), but that did not make me a fluent speaker.
And I would add, to re-address the specific OP's question about Greek vs. English here: quite apart from the pronunciation, the gigantic array of confusing or confusable homophones and homographs in English, without the guidance of a ruled-based inflexional system, must make it horrendous to learn except as a native infant.