现在分词短语在句中的成分
According to this theory,a large meteor hitting the moon would melt the surface rock by the force of collision.
hitting the moon 是什么成分?定语还是条件状语呢?
书上说是条件状语，不太明白。
不大像是主补。
He was seen going upstairs.这是主补的结构。
来自《薄冰英语语法》