I can't have been.不能写成i didn't drive so fast
I can't have been.译成：我那时不可能开那么快.（表示对过去有把握的推测）.i didn't drive so fast .译成：我（过去）没有把车开过那么快.（没有推测的意思）
2课文I was doing eighty when I overtook you.不能写成i have been driving at eighty miles an hour when i overtook you .
因为I have been driving at eighty miles an hour when i overtook you .这句话的前半句,即 I have been driving at eighty 用的是现在完成进行时,这个事态表示动作过去发生延续到现在,以至于延续到将来,因此要求有与之相匹配的表示现在的时间.而when i overtook you 这个时间状语只表示过去的时间,所以句子本身又不匹配的情况.
3课文Didn't you see the speed limit?不能写成aren't you see the speed limit .在 Didn't you see the speed limit?句子中,主语的动作时see,see是行为动词,在转换成问句或否定句是要用助动词do,does,did等,而不用系动词be,因此本句不能用aren't.
4课文 He wasn't dreaming,officer.不写成 he didn't dream,officer .因为He wasn't dreaming,officer.如果补充完整应是：He wasn't dreaming when you overtook us,officer.所以要用过去进行时.
5课文But you'd better not do it again!和you'd better not drive so fast next time 的句意有差别,again：再一次,又一次.next time：下一次.还有原文用do it是为了避免重复drive这个词,所以还是不行.
6课文Thank you.I'll certainly be more careful.句中的careful是个形容词,形容词应该跟在be的后面.carefully是副词,副词常常用来修饰动词,形容词和副词.所以本句中只能用careful.certainly的位置比较灵活,可以在句首,句中,句尾.