Now when it comes to subjects that aren't easily studied through flash cards – like math or even a sport like skateboarding – it's harder to use a rigid spaced repetition algorithm.

现在，当涉及到那些不容易通过抽认卡进行学习的科目时——比如数学，甚至像滑板这样的运动——使用严格的间隔重复算法就比较困难了。