Questions like Why do you play chess? display subject auxiliary inversion; the auxiliary verb do appears before the subject you.
In a normal declarative clause, the adverb not occurs after the first auxiliary, before the next verb in the chain of verbs (or before any modifiers of that following verb phrase):
- You do not [play chess].
- You do not [just play chess].
If we have subject auxiliary inversion, the adverb not stays in this same position:
- Do you not [play chess]?
However, if—and only if—the adverb not is contracted with the auxiliary, in this case the verb do, it gets pulled along with it to the pre-subject position:
- Don't you [play chess]?
This is exactly what is happening is the Original Poster's example:
- Why do you not [just do it]?
- Why don't you [just do it].
- *Why do not you [just do it]? (ungrammatical)
Above we see that (7) is ungrammatical because the negative adverb not is not in its normal position before the following verb phrase. It cannot move from this position because it is not contracted with the auxiliary verb.
Some writers, for example The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002), argue that don't and other negative 'contractions' are not actually contractions at all, but negative inflections of the auxiliary verb. They cite evidence such as the fact that won't is nothing like an amalgam of the words will and not, as well as the phenomenon noticed by the Original Poster in their question. Notice that although with do and don't it looks from the spelling as if we have just tacked n't onto the verb do, the vowel in don't is completely different from the vowel in do, and thus the situation is very similar to the one with will and won't.
Other environments where don't cannot deconstruct:
Unlike the situation with negative questions, where it could be argued that don't can deconstruct, but not in situ—the not must appear in a different position, not next to the auxiliary—there is one case where don't does not readily deconstruct at all. It is also a case where some people have argued that don't is actually a marker and not a verb. This is the case of negative imperatives with subjects. And this time, the phenomenon really is to do with the item do, and not to do with auxiliaries in general.
Imperatives often occur without a subject:
- Dance!
If we want to negate the imperative we can use either the item don't or the two words do not:
- Don't dance!
- Do not dance!
So far, so good. Now, imperatives can also come with subjects. These are usually either the second person singular pronoun you or a word like somebody, anybody, nobody and so forth:
- You dance!
- Everybody dance!
[ These are pronounced very differently from the case where we have a vocative expression like Bertha/you/everybody, where we would expect a separate prosodic unit (a separate tune) for the vocative:
- Bertha, dance!
- You, dance!
- Everybody! Dance!]
Now, if we want to negate an imperative clause that has an expressed subject, we need to put the word don't before the subject:
- Don't you dance!
- Don't everyone talk at once.
However, here, there doesn't seem to be any good way at all of decomposing the don't into two separate words in these cases, especially with a second person subject:
- *Do not you dance! (ungrammatical)
- *Do you not dance! (ungrammatical imperative - would be ok as question)
Either way, what made you think 'don't' could have any meaning 'of its own', as a particle or anything else? Are you suggesting 'don't' could be something other than a contraction
– Robbie Goodwin Nov 24 '23 at 22:39