At any point in history was "Why cannot...?" used as frequently as "Why can't...?" Is it even grammatically correct to say "Why cannot you do this?" I know it can be rearranged to be "Why can you not do this?," but I always presumed the contraction and the contracted phrase could be used equivalently, without changing the sentence structure. I think this pattern holds true with the other question words (e.g. how, when, etc.); however, I also know it is common to say "Who cannot do this?". In general, are there rules pertaining to the uses of contractions in questions?