句子是进行时时 前有必动词 后面的动词加ing
例如：I'm playing basketball now.
还有一种情况 动词做名词
例如：Swimming in the river is so exciting.
还有几种情况: 1 介词后,look forward to doing 2 现在进行时,be doing 3 一些固定的动词后要ing形式,enjoy doing 4 还有就是分词作状语,而且和发出动作的人是主动关系 Not knowing her address,I could not write to her .
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