I mentioned a while back about how I love calculus because it brings back old concepts, and now u-substitution is making it's return. I easily grasped the u-substitution concept when we first learned it, so the u-sub part of indefinite integrals should have been trivial, however it had been so long since I had done u-subs I completely forgot what to do with the du. Adding definite integrals was even more confusing, but I got the hang of it after some practice. It took me a while to realize how the u-interval and x-interval were connected, and to be honest, I forgot how I figured it out, but I did.