定语从句关联词who which 后面什么时候接is,have 等动词

定语从句关联词who which 后面什么时候接is,have 等动词
只需要说明白举例子就行,不用长篇复制
我的意思是为什么我看到有的定语从句的关系词后面有is was have等动词,有的时候关系词后面没有呢

is作系动词后面描述状态,定义,
如这个非限制性定语从句：
Charles Smith,who was my former teacher,retired last year.
All that is needed is a supply of oil.
have用语完成时的时候,可与be动词连用,也可与动词连用.
如：Charles Smith,who has retired last year,was my former teacher.（完成时）
Charles Smith,who has been in school for 40 years,was my former teacher.(描述)
当然定语从句都是进一步阐述被强调部分的作用.
这没有固定的规则,定语从句关系词就代表你句子中或从句中要强调的部分,比如：
That is the man who wanted to meet you.
句中who=the man,可以写成A man wanted to meet you.And that is the man.
每个定语从句都可以拆成两个独立句子.有的定语从句是主系表结构,关系词后面就是系动词is,was等,有的从句是主谓宾结构,后面就直接跟谓语动词了.有的时候被强调部分在从句中不是主语,需要转换句式,如：
Beijing is the place where (in which) I was born.
=Beijing is a place.I was born there (in Beijing).