If you haven’t heard, there has been a lot of discussion lately on the category of “deponency” in Greek. For the sake of those who have not kept up with this, let me give a really brief summary: It seems that the category is not native to Greek, but was brought across from the study of Latin to attempt to simplify apparent complexity/ambiguity in the middle voice in Greek. So people are now suggesting we do away with the category, and rather allow the Greek middle to be a genuine middle – on its own terms.
For a more sophisticated discussion of what those terms might be – and an enlightening pictorial representation of the Greek middle – see Mike’s recent post here.
And finally, a thought on how to develop your understanding of NT Greek: Just keep reading. I find that through reading Greek, some things gradually become intuitive, such that when they are explained at an academic level, they simply click.