If you have previous experience with a language and feel that lessons are too easy, there are several ways to advance more quickly, skipping ahead by unlocking grayed out skills and lessons.
1. Take a placement test. This option is available the first time you start a new language tree to make sure you are placed in a spot that isn't too easy for you.
2. If you would rather test out of many skills at once, you can click on the checkpoints (keyholes) found throughout your skill tree on the home page. Passing this test will unlock all skills in the tree up to that point.