I didn t know what was happening outside这句话中为什么happen加ing?

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Ididntknowwhatwashappeningoutside这句话中为什么happen加ing?Ididntknowwhatwashappeningoutside这句话中为什么happen加in

I didn t know what was happening outside这句话中为什么happen加ing?
I didn t know what was happening outside这句话中为什么happen加ing?

I didn t know what was happening outside这句话中为什么happen加ing?
表示过去正在进行,用过去进行时,我不知道外面正在发生什么事