I'm sure something is true about the present. (a She couldn't have forgotten her own name! / b When she answered the phone, it must have been her mother calling. / c Joe has been running for hours. He must be tired!)
C (must be)
I'm making a deduction about a past situation. (a She didn't answer her phone. She might have left on her vacation this morning. / b He can't be on vacation. I saw him at school. / c They must be really happy.)
A (might have left)
I'm making a deduction about a present situation. (a. Ben might not come today. I don't know. / b Helen must be at home. Her lights are on. / c They must have been late yesterday.)
B (A: future; C: past)
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