Past of modals - Teoría

USO:

Las formas modales COULD, MAY y MIGHT se usan con HAVE y el participio pasado de los verbos para especular o formar opiniones acerca de posibles acontecimientos del pasado. MUST HAVE se usa del mismo modo para expresar deducciones lógicas sobre el pasado. SHOULD HAVE se usa para hablar sobre expectativas, supuestos y obligaciones en el pasado.

EJEMPLOS:

'I'm worried. Tom should have been on that plane, but he wasn't.'
'I'm sure he's fine; he must have taken a later flight.'
'I don't know. He may have decided to come swimming. He might have had a problem.'