How do you earn less than 10 XP points at a time?
Nearly every time I complete a lesson or strengthen a skill I get 10 points. There was one occasion where I only got five points, I didn't notice it at the time so I don't recall where/how it happened.
You can set your daily goal as low as 1 point, but what ways are there to earn less than 10 points at a time?
You can do a Timed Practice and only answer one question right.
Are you sure you only got a five and not 10 +5 combo bonus?
As I said, I didn't notice it at the time, so I'm not sure exactly how it happened, just at one point I noticed my XP total for the next level ended in a 5 rather than a 0. I've never done a timed practice and don't know if it's available in the web-based version of Duolingo.
Yes, it is. If you want to purchase it, click on your Lingot number and just buy it. You'll be able to use it next time you strengthen a skill.