延後主語

延後主語,英文叫extraposed subject. 這句構我在中學時已經學過,例子:

 It is dangerous to swim in bad weather.  (句1)

句子的it究竟指什麼呢? 中學時我也有概念, 便是這個it 其實是代替句子的尾部,即to swim in bad weather這infinitival clause (不定式子句). 換句話說, 句子本身是這樣的:

To swim in bad weather is dangerous. (句2)

由於句2的主語太長,我們便將它放到最後,而以it 去代替其位置. 這個it, 英文叫anticipatory it或introductory it.句子文法上的主語 (grammatical subject)是it, 但詞義上的主語(semantic subject)卻是to swim in bad weather.

以上例句的主語to swim in bad weather 不是很長, 延後不延後似乎沒有特別差別;但有時候主語可以很長,例如:

It is gratifying to note that more people are realizing the harm of eating fast food. (句3)

句3的「詞義主語」是 to note that more people are realizing the harm of eating fast food. 如果用它作「文法主語」,便會「頭大身細」:To note that more people are realizing the harm of eating fast food is gratifying.  在這種情況下, 把主語延後,便可使句子更易理解.

除了不定式子句外, 動名詞子句 (gerundial clause)也可被延後:

It is heartbreaking knowing the news. (句4)

除形容詞外, 有些既定名詞 (set expression)後也會跟延後主語, 如:

It is no use crying over spilled milk. (句5)

句5的詞義主語是crying over spilled milk, 不延後的話, 句5便成:  Crying over spilled milk is no use.

That子句和wh子句也是常常被延後的主語:

It surprised me that she was expelled from school. (句6)

It is none of your business who I love. (句7)