Keep Going or Review...?

That's a great question. Maybe do a quick review of the things you don't feel you understand, and then move on? In my opinion, our goal in learning iOS should be primarily to understand how and why you do something.

I think of it like this: there's a lot of code I don't remember off the top of my head, like how to get the extension of a URL from an NSString. I think it's [_somepath pathExtension], but I'm not 100% sure. And is it something I do with a NSURL or a NSString? That I'm not sure about either. But I can find it in the class reference in about 30 seconds if I need to. Eventually I want to have those things memorized, but for now the most important things for me are the paradigms and practices: when do I declare a reference as __weak, and when do I declare it as __strong? How do I make a Singleton instance, and why does it work? How do I make an app that works for both iPhone and iPad without it looking ridiculous? What's the proper way to override a class when you make custom initializers? (Because you need to make a default initializer.)

I think you'll find that you can pick up a lot of the language nuances by osmosis and practice when you're focusing on these bigger concepts and solidifying them in your head.

I learned iOS from Aaron Hillegass' book, so I can't comment on where you should go next on Treehouse. But "Build a Blog Reader" and "iOS Foundations" look good.