http://zhidao.baidu.com/question/393261216.html
什么叫现在分词作后置定语：
The girl standing under the tree is really charming.
这句话中，standing under the tree就做girl的后置定语，意为“站在树下的女孩子” 。
定语：起修饰限定作用的语言成分，多为“···的”； 后置定语，顾名思义，放在被修饰词后的定语。
现在分词作定语， 多表主动和进行的含义。有别于过去分词，表被动含义。
请体会下两句：
The teacher came in, following some students.
老师跟着一些学生进来了。（学生在老师之前）
The teacher came in, followed by some students.
老师被一些学生跟着进来了。即：老师进来了，他后面跟着一些学生。（学生在老师之后）
[转]不定式、分词、动名词作定语时的比较
http://blog.sina.com.cn/s/blog_5f574db401014qdj.html
原文地址：不定式、分词、动名词作定语时的比较作者：李晓明
不定式、分词、动名词作定语时的比较
A．作前置定语
现在分词、过去分词、动名词 可以作前置定语，而不定式不能作前置定语。如：
① a developing country , a developed country ; boiling water，boiled water ; ②building materials , a swimming pool ; the sleeping boy , the coming new year 。
辨异的方法很简单：①现在分词具有主动、进行的特点，而过去分词则具有被动、完成的特点。例如，a developing country=a country which is developing (发展中国家 )； boiling water=water which is boiling(正在滚开的水 ) ; a developed country=a country which has developed（发达国家 ）； boiled water=water which has been boiled(已滚开过的水)。②现在分词和动名词同形，但作前置定语时，动名词表示所修饰的名词的用途，例如， .building materials=materials for building; a swimming pool=a pool for swimming ；而现在分词作前置定语时，表示所修饰的名词正在进行的动作，两者是逻辑上的主谓关系。例如，the sleeping boy=the boy who is sleeping ; the coming new year=the new year that is coming 。
B．作后置定语
不定式、现在分词、过去分词都能作作后置定语，而动名词不作后置定语。先看例句：
① The problem to be discussed at the meeting is very important.
② The problem discussed discussed at the meeting is very important.
③ The problem being discussed at the meeting is very important.
辨析比较：上面例句分别用不定式、现在分词、过去分词作后置定语，是因为它们存在不同的情况：不定式作定语，表示动作正要进行（未发生）；现在分词作定语，表示动作正在进行（正在发生）；过去分词作定语，表示动作已完成（已发生）。据此，我们可以把上面的例句还原为定语从句作定语的句子，这样学生就更容易理解：
①The problem to be discussed at the meeting is very important.=The problem ( which is ) to be discussed at the meeting is very important.
②The problem discussed at the meeting is very important.= The problem (which was ) discussed at the meeting is very important.
③The problem being discussed at the meeting is very important.= The problem (which is ) being discussed at the meeting is very important.