现在分词转换为定语从句的问题?

题目

现在分词转换为定语从句的问题?
3、现在分词短语做后置定语
现在分词短语做定语时,一定放在被修饰成分的后面.
例如：
They built a highway leading into the mountains.
他们修建了一条通往山里的公路.
We met a group of pupils returning from school.
我们碰到了一群从学校回来的孩子.
现在分词短语做后置定语,在意义上相当于一个定语从句.但在转换时,要注意动词的主语和时态.例如上述两句可转化为:
They built a highway which led into the mountains.
We met a group of pupils who returned from school
问：1、为什么要用现在分词?
2、转为定语从句,动词的时态又为什么要发生变化?
3、能用of属格句式吗?
They built a highway of leading into the mountains
We met a group pupils of returning from school

答案