-n't

Etymology
Univerbation of not and some preceding words (see usage notes) and following elision of medial vowel, or univerbation of n't and some preceding words. Compare.

Usage notes

 * This contraction does not receive stress, even for contrast: either you DID or you DIDn't.
 * The suffix can only be added to auxiliary verbs, including  and  in certain uses (see ), as well as main verbs  (in almost all uses) and  (in some uses). Indeed, in some dialects, not all auxiliary verbs accept ; for example,  is present in some dialects and absent in others.
 * Some verbs change their form when is added; for example,  +  is usually, and  +  is frequently  or  (though all three of these are dialect-dependent).
 * Though verbs with are usually considered contractions of versions using the adverb, grammatically they behave somewhat differently. For example, when subject and verb are inverted,  remains attached to the verb, whereas  does not:
 * Isn’t that difficult?
 * Is that not difficult?
 * Semantically, may have either “high attachment” or “low attachment”, depending primarily on the verb. For example, “I can’t leave” means “It is not the case that I can leave” (usually “I am unable to leave”), whereas “I shouldn’t leave” means basically “I should stay” (which is a narrower statement, and therefore a stronger one, than “It is not the case that I should leave”). (“I can stay”, by contrast, is very different from “I can’t leave.”) Similar variation is seen with the adverb, as well as various other negative constructions.