modal verb
BAD : If I hadn't seen the car coming, I may have been killed.
GOOD : If I hadn't seen the car coming, I might have been killed.
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To refer to something in the past that was possible but did not actually happen, use might have or could have (NOT may have ): 'If you had worked harder, you might have passed the exam.'
Compare: 'I may have left it at home - I'm not sure.'