你好 我想問什麼時候可以用relative clauses without a relative pronoun？(((急）😣20點?

Adjective clauses are introduced by relatives pronouns:-
=Nomiative
=who, that, which

Accusative:-
=whom, who, that, which,

The man to whom I spoke was Mr. Brown.-----can't be omitted
The man (whom) I spoke to was Brown-----(whom) can be omitted
The man (that) I spoke to was Brown.-----(always use THAT)

The box in which I put my money is lost !----can't be omitted
The box (which) I put my piggy-sum money in is lost !-----(which) can be omitted.
The box (that) I put my money in is lost-------(always use THAT conveniently)
When prepositions are used together with relative pronouns, they may appear in two different positions with (whom),&(which);---and may be omitted.

eg:-He was the greatest poet (that) ever lived.
eg:-The No.371 airplane (that) sank in S.China sea was never found.
eg: -The girl (that) I met last night was my former friend.
eg:-The money (that) you gave me went to the Charity, fortunately.

When relative pronoun are used accusatively in defining clauses, they can be omitted:-
eg:-The girl I met last night was my ex-girl friend.
eg:-The man I spoke to was a Mr. Brown.

限定用法的關係子句裡的「受格」關係代名詞，如whom/that等可以省略，但whom前面有介系詞時除外：
The man (whom) we saw is Mr. Brown.（括號內的字可省略）
The man (that) we saw is Mr. Brown.

The girl with whom I work is coming. （不可省略）
= The girl (whom) I work with is coming.
= The girl (that) I work with is coming.