When you’re trying to explain to a student a subject you know very well, there can be an enormous temptation to start out with “It’s easy. You just…” You must resist this temptation.
The problem is that for you, if it’s a subject that you know inside out, everything feels this way. After all, you can’t even remember a time when this stuff wasn’t easy.
So you start giving what you think will be a simple explanation. Except that part way through, you realize that it’s only simple because there’s something else that’s also simple, which you’ll need to explain first.
And then you realize that that simple thing relies on two other very simple things. And so on.
It’s not that any part of the explanation is complicated. It’s more that when you know something like the back of your hand, you forget just how many different steps you’ve gone through to attain that level of knowledge.
This may be one of the trickier things about teaching: The better you know something, the harder it is for you to remember that it’s not really easy at all, for somebody who doesn’t already know it.