Question

分析一下每个词句子成分结构

It is she who sings the best in my class

Answer 1

一楼的回答有失偏颇. 这句子是一个"主系表"结构.
It 主语
is 联系动词,做谓语
she 表语
who sings the best in my class 同位语从句,=she,用来补充说明she的状态

其实who sings the best in my class是病句,应去掉the,意思是"在班里唱得最好".

Answer 2

it是主语
is是表语
she是宾语
who sings the best in my class是宾语从句
who是从句的主语
sings是从句的谓语
the best是程度状语，in my class 是地点状语.