doing 放句首是动名词,其相当于名词，表状态。
例句：Doing lots of exercise is good for health.多锻炼对健康有益。
to do放句首为动词不定式，表目的，多用于后面所说的事情还没有发生。
例句：To be a author is my dream.成为一个作家是我的梦想。
Reading is an art.
读书是一种艺术。
Traveling by ship is comfortable.
乘船旅行很舒服。
Seeing is believing.
眼见为实。
To know oneself is difficult.
人贵有自知之明。
To say is one thing and to do is another.
说是一回事,做又是另一回事。
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