The lady who is standing behind the counter served me?请问这句是什

The lady who is standing behind the counter served me?请问这句是什 连词成句1.counter,the,behind,lady,who,the,standing,is,me,served. the lady who is standing behind the counter The lady who is standing behind the counter. the lady who is standing behind the counter请问为什么用is不用was Who served you?The man （who is）standing behind the counter.文 为什么The lady who is standing behind the counter.不能改成The lady The lady who is standing behind the counter.请问英文高手这类句子中为什么要用 1.which are on the counter 2.The lady who is standing behind [the lady who is standing behind the counter.]怎么理解这个句子? 请教此句英语语法：the lady who is standing behind the counter. 有句：“who served you the man standing behind the counter.”