How to know when an Error will arise?

Take a look at the videos covering "Optionals". Again, a very powerful concept - possibly one of the best in Swift. I can't really do them justice here in a few lines, but an optional is a way of indicating that a variable can not only contain a value (e.g. an integer with a value of 5), but can potentially also contain a non-value (i.e. nil). Optionals allow Swift to check for common runtime errors at compile time - so you get to fix the issues before your customer ever sees them! That probably makes little sense, but go read up on Optionals and you'll soon understand how powerful they are.

I've not watched that particular video, but the guard keyword is very useful as it allows you to avoid what Apple call the Pyramid of Doom where you have lots of nested if let checks trying to unwrap optionals and weed out what conditions might cause you grief. Guard flips everything around from checking for what might be wrong, with asserting what you need to be true. With the guard keyword you state the conditions required in order to continue, and if they aren't met guards else block is executed where you would typically return back to the calling function with an error condition, throw an error (via throw statement) or perform something blunt like calling fatalAlert(). It's a simple concept, and in trivial examples you don't see the full power of it, but in a non-trivial codebase it's a wonderful construct and one which developers should definitely look to taking advantage of.