请问instructing him...在句子中干什么成分？为何不是用to instruct him?The editor at once sent the journalist a fax instructing him to find out the exact number of steps....

The editor at once sent the journalist a fax instructing him to find out the exact number of steps.

目前分词 instructing...，是随着状语，也暗示肯定的目的性。表示在发传真的同时，“指示”这个动作也在发生，也就是说，两个动作是同时发生的。大家了解，不定式表示以后，目的性非常强。本句若用不定式 toinstruct...，“为了指示...才发电报”，不符合句意。

目前分词作随着状语，也总是带有目的性。比如：

She waited impatiently, hoping he could come earlier.

她不耐烦地等待着，期望他能早点来。

The professor put a finger in his mouth, tasted it and smiled,looking rather pleased.

教授把一个手指放进嘴里，尝了尝，笑了，看着非常高兴。

The secretary worked late into night, preparing a report for the manager.

秘书为了给经理筹备一个报告而工作到深夜。