Ah, ok. Yeah that will all still work, but it depends on your method of doing those things for what you will need to change.
For example, if you answer a question wrong, and present a new view that says "blah blah this was wrong...try again?" then presenting that view is exactly how I described (push the view controller on to the navigation controller, and you're done).
The question there though is how are you going back to the question? Are you actually dismissing the "wrong answer" view (dismissModalViewController) or are you re-presenting the view for that question? If you're dimissing, then you would call the cooresponding navigationController method "popViewController" wh ich would return you to the question. You have another option with NavigationControllers though which is really quite nice. When you push a ViewController onto a NavigationController you get a built in back button on the top left corner. You could literally just use that as your back button. If you didn't want that back button on the navigationBar to appear, there are various ways of removing it or hiding it.
Another option that you could do is treat each question as a "page" on your navigation controller. Think of your navigation controller as your quiz, which has 5 pages. Then for each page (question) you can present a modal view controller for the right/wrong answer. Then those two would do two things based on right/wrong answers. A wrong answer would simply dismiss the modal view controller, which would take you back to the question automatically. The right answer "next question" button would dismiss the modal view controller AND push the next question view controller (that would most likely involve implementing a delegate method though, which would let your right answer view controller tell the question view controller to move to the next view). This method of doing things could be better from a design standpoint, but the first method would definitely be easier.
To be honest a lot of this is important stuff to know for iOS development. Using the right View Controller in the right way is very important for common human interface design for the iPhone. Look at some of Apple's apps for guidance. You'll find that modal views are used sparingly, and not as the core method of presenting information. The tool designed for that purpose IS the NavigationController. I highly recommend reading more on that and using what you learn to help with tweaking your application. It sounds like you've made a lot of progress with your app, but I think considering these common design practices would go along way towards polishing it up.