主语从句与宾语从句同属名词性从句,二者有很多相似之处.我们已熟练掌握了宾语从句,因此了解二者的共性特征,掌握主语从句的难度将大大减轻.

　　(一)语序相同:都采用陈述语序.如:

　　I don't know whether he has come back.(宾从)

　　Whether he has come back is still unknown.(主从)(划线部分不可改为:Whether has he come back)

　　(二)从句与主句的时态搭配规则相同

　　1.主句用过去时,从句一般用过去时(如从句表示客观真理,可用现在时态).如:

　　I didn't know what food she bought.(宾从)

　　It was unknown what food she bought.(主从)

　　At that time people already knew that the earth is round.(宾从)

　　It was already known at that time that the earth is round.(主从)

　　2.主句为现在时,从句可根据需要选用各种时态.如:

　　I don't know when he came back/when he'll come back/whether he has come back.(宾从)

　　It is unknown when he came back / when he'll come back/whether he has come back.(主从)

　　(三)引导词相同,且在从句中的含义及句法功能相同

　　主语从句和宾语从句一样,引导词的选择都取决于该词在从句中的含义及充当的成分.各引导词功能如下:

　　(四)表示"建议"或"要求"时,从句都用虚拟语气.如:

　　They suggested that Tom (should) start as early as possible. (宾从)

　　It was suggested that Tom (should) start as early as possible. (主从)

　　二、查漏补缺,把握主语从句的独特性.

　　(一)注意主语从句与主句谓语动词的一致

　　1.主语从句多表示单一概念,因此主句谓语动词多用单数.如:

　　When and where we'll go isn't decided.

　　2.what引导的主语从句,根据表达含义的不同确定主句谓语动词的单复数.如:

　　What we need are books/is water.

　　(二)熟记"it作形式主语,主语从句后置"的常见句型结构

　　1.It + seem/appear/happen/chance等的适当形式+that从句.

　　2.It +系动词+过去分词+ that从句.

　　3.It +系动词+形容词+ that从句.

　　该句型中,如从句表示"应该做某事",从句需用虚拟语气.如:

　　It's necessary/important that you (should) be ready by 5 o'clock.

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