he __a machine the last time we saw him.(was operating)为什么不填was operating on?

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he __a machine the last time we saw him.(was operating)为什么不填was operating on?

he __a machine the last time we saw him.(was operating)为什么不填was operating on?
he __a machine the last time we saw him.(was operating)为什么不填was operating on?

he __a machine the last time we saw him.(was operating)为什么不填was operating on?
operating是及物的,操作,没必要加on

这里强调的是当我们看到他时,他正在做什么,说的是进行时。而由于又发生在过去,就用过去进行时了。