名詞性從句中what和that用法辨

名詞性從句可以分為主語從句、賓語從句、表語從句和同位語從句四種，在選用引導詞時，what和that極易被混淆，讓我們從用法上來看看兩者的區別。

（1） what在從句中擔任句子成分，that不擔任

what在引導名詞性從句時往往在從句中擔任一定成分，表示疑問、感嘆或指代“所……的人/事物”；而that引導名詞性從句時僅作引導詞，無意義，所以它引導的從句本身必定是完整的。例如：

主語從句What Mary is is a secret. （what表示疑問）That he finished the work in such a short time surprised us all.

賓語從句I don’t know what a naughty boy he is. （what表示感嘆）He said that he would go to the college the next year.

表語從句This book is what I want to tell you about. （what相當于the one that）The fact is that he has not been seen recently.

同位語從句They have no idea at all what he is working on. （what表示疑問）Word came that their team had won the championship.

（2） what在任何從句中都不能省略，that引導賓語從句時有時可以省略

由于what在其引導的從句中擔任一定的句子成分，所以任何時候都不能省略；而that在名詞性從句中只起引導作用，不擔任句子成分，在賓語從句中有時可以省略，但在其他三種名詞性從句中不能省略。

例如，What she told me is not true一句中，what在主語從句中作told的賓語，不能省略，也不能用that代替，否則句子不完整；John said (that) he was leaving for London on Wednesday一句中，賓語從句句意完整，故用that，且that可省略。

但在以下幾種情況中，賓語從句中的that不能省略。

■當賓語從句被分隔時。如I don’t doubt， in any case， that he keeps his word一句中，從句被短語in any case分隔，that不能省略。

■當賓語從句本身為復合句時。如He told me that if he was free he would drive me there一句中，從句if he was free he would drive me there本身為復合句，that不能省略。

■當賓語從句的主語為that時。如The man said that that was really an adventure to him一句中，從句的主語為that，為避免混淆，引導詞that不能省略。

■當有并列的賓語從句時。如Everyone could see what was happening and that poor Tom was nearly frightened一句中，動詞see的賓語是由and連接的、分別用what和that引導的兩個并列的從句，that不能省略。如果省略that，整個句子就會成為由and連接的并列句，句意與原句不同。

（3） that引導的主語從句可以后置，what引導的主語從句中只有表示疑問含義的可以后置