The engine started quickly,to my relief,but loudly,roaring to life and then idling at top volume.

题目

The engine started quickly,to my relief,but loudly,roaring to life and then idling at top volume.
此句中roar和idle为什么用ing形式?

答案

此句中roar和idle为什么用ing形式?是因为用动词表示主句的伴随状态.比如,
He rushed into the room,crying.他哭着冲进屋子.
原句中的roaring 和 idling 是说明 engine started 时的情形.