Use must when you're almost certain something is true, and can't when you're almost certain it's NOT true. Use might / could when you're less sure: She must be tired. That can't be right. He might have left.

Use must when you're almost certain something is true, and can't when you're almost certain it's NOT true. Use might / could when you're less sure: She must be tired. That can't be right. He might have left.