The
Past Form of Should:
To form the past of should simply add have + past participle:
--I
missed my plane this morning. I should have left earlier.
--I
can't find a hotel room. I should have made a reservation.
The
past form of should conveys the notion of a failure or omission.
Note that the customary pronunciation of should have is should've
or shouda. The negative form is should not have, pronounced in colloquial
English as shouldn't've or shouldn't'a.