同位语从句和定语从句很相似，但还是有区别的，区别主要在以下三方面：

　　1. 从词类上区别

　　同位语从句前面的名词只能是idea，fact，news，hope，belife，suggestion，proposal

　　，word，thought,doubt,truth,possibility,promise,order，等有一定内涵的名词，而定语从句的先行词可以是名词、代词，主句的一部分或是整个主句，如： The possibility that the majority of the labour force will work athome is often discussed。人们经常讨论大多数劳动力将会在家里工作的可能性。(同位语从句)

　　Our teamhas won the game, which made us very happy. 我们的队赢了，这让我们很高兴。(定语从句)

　　2. 从性质上区别

　　定语从句是从句对其先行词的修饰或限制，属于形容词性从句的范畴；而同位语从句是从句对前面抽象名词的进一步的说明和解释，属于名词性从句的范畴，如：

　　The newsthat our team has won the game was true. 我们队赢了那场比赛的消息是真的。(同位语从句，补充说明news到底是一个什么消息。)

　　The newsthat he told me yesterday was true. 昨天他告诉我的那个消息是真的。(定语从句，news在从句中作told的宾语。)

　　3. 从引导词及其在句子中的成分上区别

　　有些引导词如how,whether, what可以引导同位语从句，但不能引导定语从句，如：

　　Thatquestion whether we need it has not been considered. 我们是否需要它这个问题还没有考虑。(同位语从句)

　　引导词that引导定语从句时，在从句中一般作主语或宾语(指物时还可以用which代替)，并且作宾语时常常省略，that在同位语从句中仅起连接作用，不充当任何成分，并且不能省略，也不能用which来代替，如：

　　Theorder that we should send a few people to help the other groups was receivedyesterday. 我们应派几个人去帮别的几个小组的命令昨天收到了。(同位语从句，是对order的具体解释，that虽不作成分，但不能省略。)

　　Theorder that we received yesterday was that we should send a few people to helpthe other groups. 我们昨天收到的命令是我们应该派几个人去帮助别的几个小组。(定语从句，是名词order的修饰语，that在从句中作received的宾语，可以省略。)

　　【真题例句】

　　The issue of whether life ever existed on the planet,and whether it persists to this day, has been highlighted by mounting evidencethat the Red Planet once had abundant stable, liquid water and by thecontinuing controversy over suggestions that bacterial fossils rode to Earth ona meteorite from Mars。

　　【解析】

　　句子可拆分为：The issue of //whether life everexisted on the planet, and whether it persists to this day, //has beenhighlighted //by mounting evidence //that the Red Planet once had abundantstable, liquid water and //by the continuing controversy over suggestions//that bacterial fossils rode to Earth on a meteorite from Mars。

　　主句为：Theissue of whether life ever existed on the planet, and whether it persists tothis day, has been highlighted by… and by…。主句主语的结构为： The issue of whether…and whether…。of 短语修饰the issue, of 短语较长一般译在后面；that the Red Planet…是evidence的同位语从句，(即是evidence的具体内容)。that bacterial fossils…是suggestions的同位语从句，说明suggestions的具体内容。)第一个同位语从句可以直接翻译在所修饰词后面。第二个同位语可以放在所修饰的名词前面，充当定语。

　　【参考译文】

　　越来越多的证据表明，这个红色行星上曾经有稳定而丰富的液态水，而且人们对从火星落到地球上的细菌化石陨石的说法一直有争论，使火星上是否存在过生命和是否至今仍有生命的问题成为了引人注目的重点。