21. 她就是那个到明年年底，会攒一万美圆的那个女孩。
22. 你认识那个警察一直在找的小偷吗？
23. 昨天他告诉我，是谁吃了我的苹果，那个苹果是我妈妈给我买的。
24. 自从她出生以来他就从未见过她爸爸。
25. 她昨天告诉我，自从他去年见到她父亲，就再也没有见过了。
26. 你知道你们班长喜欢的那个是贼的女孩吗？

答案：
21. 她就是那个到明年年底，会攒一万美圆的那个女孩。
She is the girl who will have made ten thousand dollars by the end of nest year.

22. 你认识那个警察一直在找的小偷吗？
Do you know the thief whom (who) the policeman has been looking for?

23. 昨天他告诉我，是谁吃了我的苹果，那个苹果是我妈妈给我买的。
Yesterday, He told me who had eaten my apple which my mother bought for me.

24. 自从她出生以来她就从未见过她爸爸。
She has never seen her father since he was born.

25. 她昨天告诉我，自从他去年见到她父亲，就再也没有见过了。
Yesterday, she told me that she had never seen her father sine she was born

26. 你知道你们班长喜欢的那个是贼的女孩吗？
Do you know the girl who (whom) your monitor likes?