2015年职称英语卫生类语法否定句：
他的设计肯定还没完成。
误：He mustn't have completed his design.
正：He can't have completed his design.
对比较有把握的推测，肯定句中用must，而否定句中用can't，对过去的推测，在must和can't后接完成形式。
2015年职称外语等级考试报名指南
