How do I get bonus slots?
I want to learn Spanish idioms, but I can't. How do I get this lesson? I have enough lingots.
Because they require some knowledge, you must have all of the skills (with all of their lessons) complete to that point in your skill tree before you can purchase the bonus lessons. Check that and please let us know if you're past that point so the staff will know if they need to look for a problem.
Excellent! Finish those three and you should be able to unlock the bonus skills. The empty bonus slot was for Christmas last year, so most of us expect that it will be back again soon, though a Halloween bonus was recently suggested that sounded like another great seasonal bonus to me.