21. This is my pen, isn't it?(陈述部分的主语this ,that,简略问句用it:陈述部分的主语these, those,简略问句的主语用they.)

20. That China is a great socialist country is well known, isn't it?(不定式短语，动名词短语或主语从句作主语，简略问句的主语用it.)

19. She dislikes smoking, doesn't she?(如果陈述部分里出现表示否定意义的某些词 如never, hardly, few, little, nothing, nobody等，简略问句要用肯定式；但是， 如果谓语是通过加前缀构成的否定词 ，简略问句用否定。)

18. Tom has been reading novels, but he didn't read this afternoon, did he?(陈述句是并列句， 简略问句的主谓语应和后一个分句一致。)

17. I wish to go to the cinema, may I?(当陈述句是主语结构I wish, 用以征求对方意见，简略问句用may I.)

16. There is something wrong with your TV set, isn't there?(陈述部分为there+be结构，疑问部分用there,省略主语代词。)

15. I am a three-good student, aren't I?(陈述部分用I am 时，疑问部分用aren't I.)

14. Nothing can stop us now, can't it?(陈述部分用everything, nothing作主语时，疑问部分主语用it.)

13. You had to do it well, didn't you?(陈述部分的谓语包含have to, has to, had to时，疑问部分要用do 的适当形式。)

12．We usually have breakfast at seven, don't we?(当have不作"有"解而用作行为动词时，简略问句用do 适当形式)