1.He could have passed the exam, but he was too careless.§2.I couldn't have passed the exam without your help.§3.It was an easy test and he should/ought to have passed, but he didn't.§4. You needn't have woken him up at 5 o'clock. It's too early.§5.I would have come to your party last night ,but my car broke down on the half way.