Maya's laptop died before a Saturday call. Theo came to help. Every plan he names is already in the past.
Modal, have, participle
Should, could, or would, then have, then a past participle.
I should have saved it.
I could have come.
I would have worked.
I should save it yesterday.
Never should had
The middle word stays have. It does not become had.
I should have saved it.
I should had saved it.