那么，具体地说，什么是"双重"?托福语法部分常见的具有"双重"功能的连词有哪些呢?所谓的“双重”，这里指的是连词除了正常的句法功能外还充当另外一个句子成分。而所谓的"正常”指的是按照一般的语法规则要求。在这种"正常"的情况下：

　　①形容词性(定语从句)的连词(关系代词)除有“关联”功能外还在从句中充当一个实际成分(主语、动词宾语、介词宾语或定语)。如：
　　The same forces that create surface tension in any sample Of water are 
　　responsible fOr the maintenance Of these unbroken columns Of water．(充当主语)
　　The early European settlers Of the United States did not abandon the diversions with which their ancestors had traditionally relieved the tedium of life．(充当介词宾语)

　　但涉及到时间、地点(方位)、原因、方式时，连词(关系代词)不充当从句中的成分.如：

　　The knee is joint where the thigh bone meets the large bone Of the lower leg．

　　②名词性从句(主语、宾语或表语、同位语从句)和副词性从句(状语从句)的连词(关系代词)

　　除具有这一功能外，在从句中一般不充当任何成分。如：

　　That he was an honest man helped him win the respect from the people around．
　　Articles differ from editorials in that articles present facts while editorials present opinions．
　　根据对过去托福考题的总结,托福语法中的总结,托福语法中的"双重"连词有三种类型"

　　①.what

　　充当(主句和从句——下同)两个主语、两个宾语、一个主语十一个宾语。例(18)中的"what'’即是充当了两个主语。又如：
 
　　I believed what he said. 句中的"what"即是充当两个宾语。

　　有时，“what'’的这种特点也成为改错部分的“题眼”。如：
　　(19)(Some) psychologists believe (what) even a person
　　(suffers) from amnesia，some (memory) remains in the unconscious．
　　②whatever，whoever，whichever
　　上述三个词的用法与“what”差不多：意义上表示一种强调。但在考题中出现的频率不及"what”高。

　　③as，than

　　本来“ss”和"than"作为关联词只有连接主句和状语从句的作用，但在托福语法考题中如果符合上面提到的“省略”要求的两个条件——主句的主语与从句的主语逻辑上相一致；从句以“be'’的适当形式作谓语的“主一系一表”结构或“类主一系一表”结构——这两个词除“正常”的连接作用还充当从句的主语：如：
　　John’s father was such a stingy man that he seldom spent On his son as much as was necessary．．
　　Mark was delighted tO see his new flat Was larger than was expected．．．
　　上面的两个句子中，“as”和“than’分别充当了从句的主语。

　　三．变性

　　“变性”是指"after"和“before”所引导的从句在从句的主语与主句的主语一致的时候可以由从句变为介词(before、after由关联词变成介词，动名词短语作介词宾语)。如： 

　　Before he became the 39th President Of United States，Jimmy Carterhad been the governor Of Georgia．

　　变成： Before becoming the 39th President Of…

　　四.变形

　　“变形”指的是"because"、“since"、“as”和“for'’引导的原因状语从句在从句的主语与主句的主语一致的时候由从句变成分词短语。如：

　　AS she was the first woman to serve in this local court，she enjoyed a greater popularity than her male colleagues。
　　变成： “Being the first woman tO serve．．．"