In case you missed this past Sunday's live stream, here's a full edited review of the class.
We learned about the grammar form ~아/어/etc. 있다. This form is used to describe the state of something using a verb. For example, it can be used to show that someone is "in the state of sitting down" or a door is "in the state of being open." It's useful with a lot of action verbs to describe something as it already is, and not just to say that something is currently happening.
We also learned how this form is different from ~고 있다, which we learned in a previous live stream.