I saw an old man and the dog

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填 that were walking.定语从句的先行词是the old man and the dog,有人又有物,关系代词要用that；主句的谓语是过去式saw,定语从句的谓语动词用过去进行时were walking.
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======以下答案可供参考======
供参考答案1:
I saw an old man and the dog( walking in the street ).
see sb doing sth 固定用法 ' 看见……在做什么）