填空题 The lights were still on. They 1 (forget) to turn them off when they left the classroom.

  • 1、
【正确答案】 1、must have forgotten    
【答案解析】句意：灯仍亮着。他们肯定是在走的时候忘了关。注意must have forgotten 实际上是一种极其肯定的判断形式，即情态动问must后跟have done 这种结构表判断。