I'm stuck, why can't I open the next skill?

You can "unlock" the next row of skills in your course when you've completed every lesson up to and including your current skill. Open an unlocked skill to refresh your learned lessons and complete the skill by passing all of its lessons.

skill example

Finishing lessons 2 and 3 from the "Phrases" skill in this example, would unlock the next row of skills with new learning content.