Question

The assistant __her did not like the way she was dressed.serving ,served 为什么不行呢?

Answer 1

其实前面的两位朋友都已说得非常清楚.这是现在分词、过去分词作定语的问题：
1.The assistant serving her = The assistant who sverved her ——服务她的那个店员
2.The assistant served （by） her = The assistant who was served by her
被她服务的那个店员
很明显,用 served 的话,后面还要加 by,否则句子是不通的.—— 如果真要表达 2.的意思.
原句要表达 1.的意思,所以要用 serving,类似的例子：
The boy standing over there is Tom's brother .站在那边的男孩是汤姆的兄弟.
The book written by a student has been published.由一个学生写的那本书已经出版了.