You might as well just stop looking for applications that will help you learn because you have now found Kahn Academy and they've got you covered - when I say covered I mean covered!
This application allows you to view over 10,000 instructional videos and articles, on just about every course you can imagine, from the most basic math to college level. All of it is completely free at your fingertips. I am not overstating it. It's got your usual mathematics and advanced math, like algebra and calculus, in fact over 150,000 Common Core aligned math exercises are included with instant feedback and step-by-step hints. It's got finance, business, healthcare and medicine, economics and history and more. It's even got videos on art history!
Once you have set up an account with Kahn Academy, you can get credit for watching videos, track your progress and see your acheivements. The app allows you download videos and view when ever you want. Using an iPad gives one subtitles and the ability to skip ahead, or go back to an earlier point.
If you're trying to learn, study or just brush up on some knowledge, I can't think of a better app for you.
Compatible with iPhone, iPod touch, and iPad.