你可以分寫兩個(gè)句子來理解一下,
He is no longer a theif.
He used to be a thief.
如果對(duì)第二句中的a thief 提問的話,應(yīng)該問what did he use to be?而不是 who did he use to be?
然后嘗試合并一下這兩個(gè)句子,就得到了He is no longer what he used to be.
而He is no longer the person who/that heused to be.這一句其實(shí)是一個(gè)表語從句中套有一個(gè)賓語從句,the person是表語,而后面的who/that heused to be是the person的定語從句.
而The question is who will travel with me to Beijing tomorrow.這個(gè)句子之所以看起來很像你寫的那個(gè)句子,其實(shí)不然,這個(gè)句子的主語是question,也就是說問題是,是什么呢,就是who 引導(dǎo)的那個(gè)問句.舉個(gè)例子來說,比如上課老師問你這樣一個(gè)問題：“who will travel with me to Beijing tomorrow.”然而你沒有挺清楚,你可以問你的同桌：“what's the question?"這時(shí)你同桌給你的答案就是The question is who will travel with me to Beijing tomorrow.
看起來你對(duì)各種從句還不是很清楚,可以找一些不同的從句自己多分析分析,理解一下.