Specifically when the surface structure uncontracted would be ungrammatical. Eg: "Don't turtles live forever?" (Do turtles not live forever/*Do not turtles live forever).
EDIT - Sorry if it's broad and vague, perhaps I simply have a poor grasp of x-bar theory in general ^_^' What I mean is, what's going on with the negation in a sentence like this? We know that it can't be put before the subject as in *Do not turtles live forever, and we know that when we say "Don't turtles live forever?" we really mean "Do turtles not live forever?" so why is the contraction "Don't" acceptable? If the surface structure was "Do turtles live forever" it would seem as simple as moving the dummy do into the CP head position, but the contraction/negation thing throws me off.