To complete a course, you have to make sure all topics of that course are marked as complete. If a topic is followed by a quiz, you can mark a topic as complete by completing that quiz (and reaching a score of 80% or higher). If the topic is not followed by a quiz, you can mark the topic as complete by clicking the green "Mark complete" button at the bottom of the topic. You can easily see which topics of a course are marked as complete in the course overview on the right-hand side. In the image below, the topic at the top is marked as complete (indicated with a check mark), and the other topics have not been marked as complete yet.