A：如果你將第二句的 that 拿掉，句子就變成 The news the president was assassinated is true. (總統遭暗殺的消息是千真萬確的)，完全不合英文語法，所以 that 不能省略。在此，”that” 是連接詞，引導 “the president was assassinated” 構成名詞子句，做為 “the news” 的同位語。當 that 所引導的名詞子句做同位語時，that 不能省略。至於 It is obvious (that) he is right. (顯然地，他是對的)，這句的 that 也是連接詞，引導 he is right 構成名詞子句，是本句的真正主詞，而 it (= that he is right) 是形式主詞，而在這種以形式主詞代替真正主詞並將真正主詞置於主句的動詞之後或挪到句末的句型中，that 可以省略；但若將真正主詞置於主句的動詞之前 (亦即下面所講的第1種情況)，那麼 that 就不能省略。
關於 that 不能省略的情況，常見的應該還有下列幾種：
- that 所引導的名詞子句當句子的主詞時：That he came late made his teacher angry. (他遲到一事讓老師生氣)。
- that 所引導的名詞子句當主詞補語時：My idea is that we should get more people to do it. (我的想法是，我們應該叫更多的人來做這件事)。
- 在所謂「分裂句」(cleft sentences) 的強調句型 “It is/was… that” 中：It was they that cleaned the classroom yesterday. (就是他們昨天打掃教室) (在此，that 也可用 who 來代替，但僅限被強調的元素是人)；It is English that Professor Chen teaches us. (陳教授教我們的課程就是英文)。
- that 所引導的名詞子句當主動詞的受詞，但這個子句被挪到句末，而其前面用 “it” 來做形式受詞時：They found it difficult that they would finish their work in two days. (他們覺得要在兩天內完成他們的工作是有困難的)；He made it clear that he was not interested in this subject. (他清楚地表示，他對這主題沒興趣)。
- that 為關係代名詞，在關係子句 (形容詞子句) 中做主詞時：That is the same man that asked me for money yesterday. (那個人就是昨天向我要錢的男子)，但 that 在關係子句中若做受詞，則可省略：The letter (that) I received was from my father. (我收到的那封信是我父親寫來的)。