表示和过去相反的:
shoule have done过去本应该做而没做的
eg.You didn`t pass the exam.You should have studied hard last year.
might have done过去本可以做而没做的
eg.It was dangerous for you to climb to the top of the building.You might have fallen off.
could have done过去本以做而没做的
eg.You were so careless to have the pan of oil burning.You could have had the whole house on fire.
needn`t have done不必做而做了的
eg.I forgot today is sunday and went to school in the morning.In fact,I needn`t have done that.
