Recently I noticed that there are some sentences which contain "can't" that sound wrong when you replace "can't" with "cannot." Here's one example. The sentence
Why can't I do it?
sounds correct. But replacing "can't" with "cannot" yields this sentence
Why cannot I do it?
I don't know if this sentence is breaking any formal grammatical rule but it just sounds very "wrong" to me (and searching for the phrase on google seems to back up the idea that it's very rarely used).
I was fairly surprised when I realized this. I think I had previously assumed that if I take any sentence containing a contraction and expand the contraction then the sentence should remain valid. But that does not seem to be the case in this example.
So here's my question:
Why is this? Did sentences like "Why cannot I do it?" used to sound more normal but they eventually died out while "Why can't I do it?" survived? Is it that "Why can I not do it?" is the proper expansion of the contraction? If so, how did the "not" end up jumping over the "I" to form the contraction? More generally I would appreciate any explanation about the origin of this phenomenon.
Best Answer
I would second the answer to this question that points out that constructions analogous to "why cannot I" were common through the 18th century and beyond, so though they sound old-fashioned today, they haven't always been ungrammatical.
But there's no real reason to insist that contemporary English and 18th-century English have to have identical grammar rules. In fact, Cambridge Grammar of the English Language (p. 91) argues that in contemporary English, can't and won't are independent words that are "negative inflections" of can and will, not true contractions. This argument is based largely on the observation that can't and won't can be used in places where cannot and will not are obsolete. (Negative inflections are uncommon in European languages but exist in other languages such as Japanese: arimasu = "to exist" and arimasen = "to be nonexistent").