Question

we watched the house being destroyed 。为什么不加was?

Answer 1

we watched the house being destroyed 

不加was，是因为

看到（观看）某某事情 正在发生 是一个结构。

本来是the house was being destroyed，但是因为有了句子的谓语动词watched，所以一个独立的句子一般只有一个谓语动词。就把the house was being destroyed变成 非谓语动词 形态了。

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Answer 2

加上was不就变成了we was watched the house being destroyed吗？
was watched是被动语态，“我们被看……”，但实际语境应该是主动。
"我们看到房子被毁掉了"。
所以不加 was.