Tip: When a registered user marks a course topic as complete, they will be taken to the next topic automatically. If you mark an entire lesson completed, it will take you to the next lesson, even if you haven't finished all the topics. You can then move to the previous and next items you've already read.
Marking a lesson or topic page as complete will also show you your progress (users need to be logged in for for this progress indicator), and allow you to keep track of where you left off.
Forums can be used to ask questions for help on any sections you are having trouble to understand, or any exercise which has not worked for you as expected.
Please read and search the book first before trying commands from an exercise or topic, and before posting questions. If you are still having issues, then post your question in detail on the forums, along with the command syntax you are using as well as any error output you see on your console.