Question

she is the only one of the girls of her class who —- the piano well 为什么空格里用单数呢

先行词不是the girls吗

Answer 1

关系代词做主语时，谓语动词也要与先行词保持一致。
先行词为one+of+复数名词，英式英语要求定语从句的关系代词应和名词的复数保持一致，即定语从句的谓语用复数形式，而美式英语要求关系代词与one保持一致，谓语动词用单数，如果先行词为the one of+复数名词或the only one of +复数名词时，定语从句的关系应 和the one或the only one保持一致，即定语从句的谓语动词用单数形式，先行词为he ,I等代词对其关系代词的谓语分别用is和am,总而言之，关系代词做主语时谓语用单数还是复数取决于它的先行词是单数还是复数。
例 She is one of the students who study English hard. 她是努力学习英语的学生之一。
John is the only one of the staff members who is going to be transferred.
约翰是职员中唯一的将要调动的人。
It is he that is studying Russian, 就是他在学俄语。

Answer 2

去掉修饰成分后，该句为：she（主） is（系） the only one（表）.of the …分别是两个介词短语作后置定语修饰the only one，who…也是修饰它的定语从句，所以用单数。

Answer 3

she 吧