We use "must have", "can't have" and "might have" in the same way as the present perfect - the action we are describing happened, or did not happen, in the past and is still true in the present. "must have": we believe the action definitely happened. "She must have left the house by now; it's nearly 11 o'clock."
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Answer:
We use "must have", "can't have" and "might have" in the same way as the present perfect - the action we are describing happened, or did not happen, in the past and is still true in the present. "must have": we believe the action definitely happened. "She must have left the house by now; it's nearly 11 o'clock."