they _saw_(saw) a boy_fall_(fall) into the river while they_

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they _saw_(saw) a boy_fall_(fall) into the river while they_were passing_(pass)it yet.讲详细为什么

句子意思是当他们经过的时候看见一个男孩掉河里了
根据句意,本句用过去式较合适
所以是they saw (saw 是 see的一般过去式)
看见某人做某事 see sb do sth 所以fall 应用原形fall
while 后面的从句通常用进行时态,所以是过去进行时 were passing
再问： see sb do和see sb doing 有两种形式，这里为什么是原形？
再答： fall 是短暂性动词，掉下去的一瞬间动作就完成了如果用falling 进行时，说明动作一直在延续，一直掉下去？不合实际。。所以用原形比较好

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