5．(2016·四川高考)However, it stayed with her and learned about the ways of the forest.Then, after two and a half years, the mother ________ (drive) the young panda away.

　　解析：drove　此处讲过去发生的事，所以用一般过去时且为主动形式，故用drive的过去式drove。

　　6．(2015·全国卷Ⅱ)This cycle ________ (go) day after day：The walls warm up during the day and cool off during the night and are thus always a timely offset (抵消) for the outside temperatures.

　　解析：goes　此处指这种循环日复一日地不断持续下去，由冒号后内容的时态可知这种情况是客观性陈述，故用一般现在时。

　　7．(2014·全国卷Ⅰ)In 1969, the pollution was terrible along the Cuyahoga River near Cleveland, Ohio. It ________ (be) unimaginable that it could ever be cleaned up.

　　解析：was　根据"In 1969, the pollution was terrible"可知，此处叙述的是过去发生的事情，因此用一般过去时，由于主语是It，因此此处谓语用was。

　　Ⅱ.单句改错

　　1．(2018·全国卷Ⅰ)During my last winter holiday, I went to the countryside with my father to visit my grandparents. I find a big change there.________________

　　解析：find→found　根据第一句中的时间状语During my last winter holiday可知，此处叙述的是去年寒假中的事情，故第二句中的谓语动词应用一般过去时。

　　2．(2018·全国卷Ⅱ)I didn't realize how right my parents are until I entered high school.________________

　　解析：are→were　根据主句的谓语动词didn't realize和until引导的时间状语从句的谓语动词entered可知，此处叙述的是过去的事情，应用一般过去时。

　　3．(2018·全国卷Ⅲ)It was Monday morning, and the writing class had just begin.________________

　　解析：begin→begun　根据语境：写作课刚刚已经开始，再结合前面的had，表明这是一个过去完成时的句子，因此把begin改为 begun。

4．(2017·全国卷Ⅰ)Before getting into the car, I thought I had learned the instructor's orders, but once I started the car, my mind goes blank.________________