Ask HN: How do you learn complex/difficult material online?

I'm trying to go through some archived Coursera courses. Sometimes I have questions about the math in the slides or the lecture and I want to ask questions but I'm not sure where to direct them.

The discussion forums aren't active anymore since the courses are archived. Sometimes I use StackExchange but it doesn't always guarantee answers. Also sometimes people answer questions there in terms of other complex topics that you don't necessarily know about.

So what I want to know is, when you're studying something complex or technically hard to understand, how do you get your questions about the materials answered?