may / might have done表示對已經發生過的情況的肯定推測，語氣稍弱。might have done還可表示“本可能做某事而實際上沒有做”。如：

　　She didn''t come on time. She may / might have missed the bus.

　　He might have given you more help, but he was very busy then.

　　should / ought to have done表示“本該做某事而實際上沒有做”，其否定形式表示“本不該做某事而實際上做了”，通常含有責備的意思。如：

　　The flowers have died. I should / ought to have watered them often.

　　You shouldn''t / ought not to have told them the truth.