不定式作表语,说明、解释主语的具体内容,这时主语可以是名词、名词性从句、不定式短语等。
The term's task was to mark the boundaries of the new city. 这一小组的任务是划定新城市的范围。
What he wanted was to finish his works as quickly as possible. 他的愿望是尽快完成他的著作。
To save time is to lengthen life. 节约时间就是延长生命。
不定式作表语时一般带to， 但是如果主语部分包含有一个表示动作的do，则to 可以省略。不带to 的不定式作表语时,句子主语最常见的形式是:
A. All + 定语从句
When I asked him about his moon rocket， he said， “All I'm trying to do is get things off the ground. ” 当我问及他的登月火箭时,他说:“我所做的一切都是为了使物体飞离地球。”
B. what 从句
What I really wanted to do was swim across the channel. 我真正想做的事情是游过海峡。
C. 形容词最高级/only/one + 名词 + 定语从句/to do
The first thing he did on his return to China was tell me the good news. 他回到中国所做的第一件事就是告诉我这好消息。
The worst thing to do is try to read your speech. 最糟糕的是照着稿子念。
