Seeing a boy lying on the ground ,I stopped the car at once

这不是复合句,是 时间状语+陈述句
复合句的构成要件是有主句和从句.本句中,现在分词短语起时间状语的作用,但并非句子.
如果把分词短语改写一下,就是主从复合句了：
when I saw a boy lying on the ground,I stopped the car at once.
此时,前面是状语从句,后面是主句.
供参考.