What if I loose all my attempts ?
That sucks a little, cos I have a lesson for which I've done all the practice, so I'm done with it, but I've also (foolishly) used up all my three attempts to master it, so I'll never have it mastered. I think the three attempts should be kept for when you're trying to skip a lesson, but for mastering a lesson we should have a separate test that we can take as many times as we want until we pass it.
Right, ok, I got a bit tangled up in the terminology - so yes, I can finish all my lessons, but a section will not count as mastered unless you take the test that you can only take three times (which is annoying).
No. You master a lesson when you have enough points gained for it. There are three ways to get the points:
1) do the refresh sessions (click the blue book that now says "Refresh this skill" instead of "Lesson [number]")
2) translate some sentences from the web (you can check what kind of points you get for any sentence by hovering the coin to the left of it)
3) use test out option. And only this way has a limited number of attempts.
In some cases I found that I had gone over the number of points needed to get to "learned" or "mastered" in some sections, just from the translations I was doing, via the "Translations" menu at the top of the page. Sometimes it felt as if I was cheating, as in some of the translations, I wasn't paying attention to the grammar.
I would like to point out that some Duolingers have found a pretty nasty way of mastering a lesson : they go to translations, write "lofjsfjqjfh" (or similar, sometimes even shorter !) in the frame, get their points and just go away. What's the point of mastering a lesson by overtly cheating?