The girl with whom I work is coming.
= The girl (whom) I work with is coming.
= The girl (that) I work with is coming.
I know the man who visited you. 當子句主詞的who不可省
＝I know the man that visited you.
The man (whom) we saw is Mr. Brown.
= The man (that) we saw is Mr. Brown. 括號內的關係代名詞whom/that可省略。
The fish which has bright color is cute.這裡的which／that是子句的主詞，不可省略。但你可將子句改寫成「分詞片語」： The fish "having bright color" is cute.或「介系詞片語」：The fish "in bright color" is cute.