改错.下列句子各有一处错误,找出并改正

1、Your mother hasn't taught you to use it ,have you（改为has she）?
2、That wasn't a knock at the back door ,was that(改为it)?
3、There is plenty of fuel in the machine ,isn't it(改为there)?
4、Bob told an interesting story to his classmates,did（改为didn't） he?