It's the objective case of they.

是they的宾格。

" Him" is in the objective case, making clear that it is an object (it is not " he" ).

“他”在客观情况下，明确表示是客体（不是“他”）。

Linguist Joseph Emonds argues that English speakers have a natural inclination to use the objective pronoun case in conjunctive clauses.

语言学家约瑟夫·埃蒙兹 (Joseph Emonds) 认为说英语的人天生倾向于在连词从句中使用宾语代词格。

And would anyone really say, " To whom do you think you're talking? " instead of " Who do you think you're talking to? " Speaking of whom, it is technically the objective case for the nominative who.

真的会有人说， “你认为你在和谁说话？ ” 而不是“你认为你在和谁说话？ ”说到whom， 从技术上讲， 这是主格who的客观情况。