The Past Form
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.