I think it would be very helpful if we could save the questions we like to revisit in the future. While I may be able to pass a test, I don't feel comfortable leaving some wrong answers behind. Sentences in some of those questions are also very important for learning the language as well I think
I would like to bring this one up; I think that would be a great contribution, and I doubt it is difficult to integrate into the system. One might put a star next to the question, and the user could add it to his/her favorites by clicking on the star.