Exercises is an amazing free resource I highly recommend. It provides lessons that can provide a refresher on some of the concepts presented here, and also a large number of interactive exercises that you can use to drill yourself and build your skills.

In this course, I incorporate exercises from and intersperse them with my own lectures, handouts, and worksheets. Content from external sources is always clearly marked as such. I have customized the exercises to best reflect the presentation of the material you find here, but I encourage you to visit their site and check out the rest of their content as well. also makes available a number of paid apps that help support what they provide for free. Please support them if you can!