shouldn't

From should + -n't.

• (UK) IPA^(key): /ˈʃʊdn̩t/
• (Yorkshire) IPA^(key): [ˈʃʊnʔ]

• Audio (US):

  (file)

shouldn't

1. Should not (negative auxiliary^[1])
   • 1989 February 5, Colin Robinson, “You Dared Us To Dream That We Are Worth Wanting Each Other”, in Gay Community News, volume 16, number 29, page 8:

     I shouldn't be telling you all this. It'll make you angry.

Although shouldn't is derived from should not, the two are not grammatically interchangeable. While "Shouldn't I do it?" is grammatical, "Should not I do it?" is no longer acceptable. It would instead be "Should I not do it?"^[1]

• can't, couldn't, mayn't, mightn't, oughtn't, shan't, won't, wouldn't

• shouldn't've

shouldn't (plural shouldn'ts)

1. (informal) Something that should not be done.
   • 2008, Working Mother, volume 31, number 8, page 20:

     Being a list-o-maniac, I suggested we make a list of the "shoulds" and "shouldn'ts." So in the darkness of hazy sleep, I began to mentally prepare mine. The first item on the "should" side was easy: a sibling for our 3-year-old daughter.

• couldn't
• wouldn't
• oughtn't
• Appendix:English modal verbs
• Appendix:English tag questions