Ho to select the right/important lessons to go further?
Could somebody explain me when you are in the "strength-phase". How I could know which "redo"-lessons I really have to redo. All "redo"-buttons have the same color...
Do you mean to ask which skills you should practice, or what you should do to be able to progress further?
If it's the first: when you're looking at your tree, look for skills that are not golden in colour. Click on them, and on the right side there will be an option to strengthen that skill. Click on that button and you will be presented with exercises from all lessons of that particular skill. It's generally a good idea to keep all skills in your tree gold to ensure that the words get into your long-term memory, but it isn't necessary for being able to advance.
If you're unable to learn new skills, make sure that you've completed all lessons in all skills up to the one you want to learn.
Redoing any of the lessons is not necessary; you can do it if you feel like you didn't grasp fully what you were taught there.