But when he looked to the side, he saw instantly that it wasn’t a dog at all, but a wolf, quickly catching up with him.
这个句子中的catching作什么成分？是作谓语吗？注意连词but后面没有谓语动词啊！
可以理解为分词作定语，修饰wolf. but后不需要谓语动词，前面的was 是动词，相当于it was not a dog, but a wolf.
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