New language tree
With the new graphics of the language tree, you cannot tell from the icons which exercises have already been completed. And when you open an icon, all the exercises seem to be checked off in green. So how do you know which ones you've done and which ones you haven't??
You'll see that the skill has a fraction of lessons completed below (for ex 2/3) and no strength bar if there are still lessons that need to be completed in a skill. If a skill is inaccessible (because you need to complete skills before it) then the skill is greyed out. If you've completed a skill (you've done very lesson inside) you'll see the strength bar next to it.
See the below image:
The 'Numbers' and 'People' skills are completed and have a strength bar. 'Clitics' still has one lesson inside the skill that needs to be done in order to progress down the tree and unlock the next skill.