从肯定到否定：（肯定的程度由高到低到否定）
must,should,may(偶尔也会用can但不太常用,因为一般用于否定或疑问了),might,can't
对现在的推测：must/should/may/might/can't +动词原形
对已发生的动作或状态的推测：
must/should/may/might/can't +have +动词的过去分词
He must be in the house because the light is on.(他现在肯定在家,因为灯亮着.）
He must have left the house because the light is off.(他肯定离开家了,因为灯没亮.）