Informal English

verb a word used to describe an action, state, or occurrence; in this case, 'shouldn’t’ve' is a contraction of 'should not have'

adverb a word or phrase that modifies or qualifies an adjective, verb, or other adverb; in this case, 'shouldn’t’ve' functions as an adverbial contraction

pronoun a word that can function by itself as a noun phrase and that refers either to the participants in the discourse or to someone or something mentioned elsewhere in the discourse; in this case, 'shouldn’t’ve' does not function as a pronoun

• One shouldn’t’ve interrupted the speaker during the presentation.
• You shouldn’t’ve eaten all the cake before the party.
• She shouldn’t’ve ditched her friends to hang out with him.
• The company shouldn’t’ve put all their eggs in one basket when it came to investments.