B.确定用现在分词还是用过去分词作表语的方法有：现在分词作表语多表示主语所具有的某种特征；过去分词作表语多表示主语所处的状态。情感动词的v-ing 形式表示具有某种特征；情感动词的v-ed形式多表示引起某种情感。一般地说，事物具有某种特征，而人才具有某种情感，因此，分词作表语的句式可归纳为：a） Sb.﹢be ﹢v-ed ; b） Sth. ﹢be﹢v-ing.
如：a. I am very excited ; b. The news is very exciting.
但有一个词例外，那就是missing 。例如“那个男孩不见了”，我们不能说“The boy is missed .”，而是 “The boy is missing.”
二、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.