If you really want to understand something, try teaching it to somebody else. One of the joys of being an educator is being forced to explicitly articulate what you think you know.
In the course of doing that, you often find that you have taken shortcuts, skipped steps, taken things for granted. But when you’re teaching the same concept to someone who hasn’t seen that concept before, you do not have that luxury.
For anyone who wants to really understand a subject well, I recommend trying to teach it to somebody else. You just might learn a thing or two yourself.