Among them will be Debbie's mother,Who swam the Channel herself when she was a girl.这句话主要意思是：黛比的妈妈还是个姑娘时,黛比的妈妈也曾横渡英吉利海峡.所以我觉得这里的 herself 放错地方了.应该改成：Among them will be Debbie's mother,Who swam the Channel when herself was a girl.我知道这样改是错的,但是不知道哪里错了,求解.
Among them will be Debbie's mother,Who swam the Channel herself when she was a girl.
这句话主要意思是：黛比的妈妈还是个姑娘时,黛比的妈妈也曾横渡英吉利海峡.
所以我觉得这里的 herself 放错地方了.
应该改成：Among them will be Debbie's mother,Who swam the Channel when herself was a girl.
我知道这样改是错的,但是不知道哪里错了,求解.
你理解的意思没错，但原句并没有问题，反身代词herself在句中作的是who swam the Channel中who的同位语，表示强调。when she was a girl是时间状语从句，你改了之后herself变成时间状语从句里的主语了。
可以看看一引动反身代词用法的总结。
这句话的意思应该是：他们当中还会有戴比的母亲，当她是一个女孩是，她曾自己游过英吉利海峡。
黛比的妈妈还是个姑娘时，黛比的妈妈也曾横渡英吉利海峡。应译为： when Debbie's mother was a girl, she also had to swim across the English channel
herself 的位置没错,只是那个关系代词who要改为小写Among them will be Debbie's mother,who swam the Channel when 【herself作主语who的同位语】she was a girl.等于：Among them will be Debbie's mother,who【he...
herself是一个反身代词，可以作同位语、宾语、表语，但是不能作主语
who swam the Channel herself when she was a girl
herself在此作同位语，而she作时间状语从句的主语