關係代名詞引導的是形容詞子句. 除了在"非限定用法"之外, 必然會有"先行詞"當成其修飾的標的物.
疑問代名詞引導的, 除了疑問句之外, 就是名詞子句. 名詞子句本身就是個標的物, 它可以當主詞/受詞/補語.
Tom is a student.
I know that Tom is a student.
疑問句也可以改為名詞子句, 但是要把原來的"倒裝結構還原", 去掉助動詞,而原本的疑問詞留下來當連接詞:
Who are you looking for?
I don't know who you are looking for.
如果要把介系詞和它的受詞放在一起, 也是可以, 只是很少人這麼寫:
For whom are you looking?
I don't know for whom you are looking.
Who was he agreeing with?
[who he was agreeing with]
Who he was agreeing with was not at all clear.
That he sings well is well known.
It is well known that he sings well.
([it]稱"虛主詞", 代替真正的主詞[that he sings well])
Who he was agreeing with was not at all clear. =
It was not at all clear who he was agreeing with.
It was not at all clear with whom he was agreeing. (A)
現在(A)是一個完整句子. (B)= The manager nodded.
(B), although (A).
The manager nodded, although it was not at all clear with whom he was agreeing.
那個經理點了點頭, 但是他到底是跟誰意見相同, 卻一點也不明確.
The manager nodded, although it was not at all clear whether Tom was the man with whom he was agreeing.
那個經理點了點頭, 但是他到底是不是跟湯姆意見相同, 卻一點也不明確.