When to use "cannot" versus "can't"?

Question

When is it best to write "can't" versus writing "cannot"? Are they interchangeable in every situation?

Can't is less formal than cannot. That's the only difference. — Robusto, Aug 20 '12 at 14:27
I would personally insist on using cannot where the emphasis is on 'not': using can't in such a case would kind of drown out the more significant 'not' part. @Robusto I suspect it is not that simple (actually, hope not). Let's wait and see what the others have to say. — Kris, Aug 20 '12 at 14:34
@Robusto: there is no unawkward way to uncontract "Why can’t I". Related: Is it incorrect to say, “Why cannot…?” — RegDwigнt, Aug 20 '12 at 14:34
Related to what @Kris says: How can I distinguish “can” & “can't” from pronunciation? — RegDwigнt, Aug 20 '12 at 14:36
@SF "Why can't I" => ??? it's not "*Why cannot I?" The best I can think of is "Why is it I cannot?" — Matt E. Эллен, Aug 22 '12 at 08:59
@Matt Эллен: "Cannot" is already a contraction of "can not" (it is, isn't it?), so uncontracting it one level more (and then splitting normally) seems okay to me. Replace "Can't" with "Don't" and uncontract that. — SF., Aug 23 '12 at 15:17
@SF I'm formulating my question in the form of an hypothesis:
Does "Why can I not have some bacon" sound awkward, as it is, in formal circumstances, only in fringe cases applicable? For example: "You can not have bacon because it is out of our reach." While for a denied request one would use "Why may I not have some bacon?" The use of 'can' instead of 'may' is already an informal speech pattern. Moreover, a denied request is unusual in formal settings, questioning it would be quite awkward per se. — gschenk, Aug 09 '16 at 04:10

Mark Beadles · Accepted Answer · 2012-08-20T15:31:59.560

Grammatically, you can use can't instead of can not or cannot in the majority of circumstances. There is an exception. In wh-movement, the contraction should not be expanded unless you also change the word order:

Why can't I have some bacon? //OK
Why cannot I have some bacon? //not OK, archaic
Why can I not have some bacon? //OK again, although formal

Stylistically, the choice between can't and cannot is more complex. Generally, people use can't in speech and informal writing, and cannot or can not in formal writing or very formal speech. Also (as @Kris points out in a comment), cannot might be used when you need to carefully distinguish it from can't in speech.

BTW, the OED says that the one-word construct cannot is "the ordinary modern way of writing can not" with a space between it, which apparently is archaic, or non-modern, or some such. — tchrist, Aug 20 '12 at 19:22

When to use "cannot" versus "can't"?

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