to do 表示目的
doing 与be动词连用表正在进行
eg.I am doing my homework.
doing 若单独使用表伴随状态
eg.I wait outside the door,playing with my hair.
doing 有时可充当名词的作用，称为动名词。
Swimming is interesting.
doing 还能与一些动词构成固定搭配
eg.mind,enjoy等词后常用doing
而to do作为动词不定式，只要表目的就能使用。
eg.I go to school to learn things.
He went to the library to borrow some books to read.
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