不少同學們在考研語法學習中經常混淆同位語從句和定語從句，對兩種從句的混淆，有時會直接影響到閱讀文章的準確理解及翻譯句子正確翻譯方法的採用，以下對兩種從句的區分方法進行專項總結，以便同學們輕鬆掌握。兩種從句的區別主要在以下三方面:
1. 從詞類上區別
同位語從句前面的名詞只能是idea, fact, news, hope, belief, suggestion, proposal, word, thought, doubt, truth, possibility, promise, order等有一定內涵的名詞，而定語從句的先行詞可以是名詞、代詞，主句的一部分或是整個主句，如:
The possibility that the majority of the labor force will work at home is often discussed.人們經常討論大多數勞動力將會在家裡工作的可能性。(同位語從句)
Our team has won the game, which made us very happy. 我們的隊贏了,這讓我們很高興。(定語從句)
2. 從性質上區別
定語從句是從句對其先行詞的修飾或限制,屬於形容詞性從句的範疇;而同位語從句是從句對前面抽象名詞的進一步的說明和解釋,屬於名詞性從句的範疇，如:
The news that our team has won the game was true. 我們隊贏了那場比賽的訊息是真的。(同位語從句,補充說明news到底是一個什麼訊息。)
The news that he told me yesterday was true. 昨天他告訴我的那個訊息是真的。(定語從句,news在從句中作told的賓語。)
3. 從引導詞及其在句子中的成分上區別
有些引導詞如how, whether, what可以引導同位語從句,但不能引導定語從句，如:
That question whether we need it has not been considered. 我們是否需要它這個問題還沒有考慮。(同位語從句)
引導詞that引導定語從句時,在從句中一般做主語或賓語(指物時還可以用which代替),並且作賓語時常常省略，that在同位語從句中僅起連線作用,不充當任何成分,並且不能省略,也不能用which來代替，如:
The order that we should send a few people to help the other groups was received yesterday. 我們應派幾個人去幫別的幾個小組的`命令昨天收到了。(同位語從句,是對order的具體解釋,that雖不作成分,但不能省略。)
The order that we received yesterday was that we should send a few people to help the other groups. 我們昨天收到的命令是我們應該派幾個人去幫助別的幾個小組。(定語從句,是名詞order的修飾語,that在從句中作received的賓語,可以省略。)
【真題例句】
The issue of whether life ever existed on the planet, and whether it persists to this day, has been highlighted by mounting evidence that the Red Planet once had abundant stable, liquid water and by the continuing controversy over suggestions that bacterial fossils rode to Earth on a meteorite from Mars.
【解析】
句子可拆分為：The issue of //whether life ever existed on the planet, and whether it persists to this day, //has been highlighted //by mounting evidence //that the Red Planet once had abundant stable, liquid water and //by the continuing controversy over suggestions //that bacterial fossils rode to Earth on a meteorite from Mars.
主句為：The issue of whether life ever existed on the planet, and whether it persists to this day, has been highlighted by… and by…。主句主語的結構為: The issue of whether…and whether…。of 短語修飾the issue, of 短語較長一般譯在後面；that the Red Planet…是evidence的同位語從句，（即是evidence的具體內容）。that bacterial fossils…是suggestions的同位語從句，說明suggestions的具體內容。）第一個同位語從句可以直接翻譯在所修飾詞後面。第二個同位語可以放在所修飾的名詞前面，充當定語。
【參考譯文】
越來越多的證據表明，這個紅色行星上曾經有穩定而豐富的液態水，而且人們對從火星落到地球上的細菌化石隕石的說法一直有爭論，使火星上是否存在過生命和是否至今仍有生命的問題成為了引人注目的重點。
