I saw an old man and the dog

问题补充：

I saw an old man and the dog

答案：

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）