I was watching a movie and heard the line "Not if you've done it before, it isn't." and it just confused the crap out of me. I'm not used to this phrase. What can it possibly mean?
It's hard to explain this fully, without a little more context, but @WeatherVane has done a good job of showing a simple, common example.
To a native speaker, it is very clear that this phrase is a retort to someone saying that a particular task is difficult. The basic answer is therefore: "No, it isn't" but then this has been expanded upon, not at the end as you might expect, but in the middle.
To rewrite this, as a stand alone, positive phrase, we would have: " is easy if you've done it before."