只有当状语从句的主语和主句的主语相同时，才能把状语从句换成短语。 否则，会引起句意改变。
While the teacher was lecturing to the class, I fell asleep.
在老师讲课的时候，我睡着了。
若改成“While lecturing to the class, I fell asleep.”，则意思是“当我在给这个班上课时，我睡着了。”这样显然不妥。
（1）从句含有be动词
如果状语从句中含有be动词的形式，我们只需要把从句主语和be动词省去，即简化成短语。简化后的短语往往是分词短语（主动的用现在分词，被动的用过去分词）、形容词短语或名词短语。这种省略主要常见于时间、地点、条件及让步状语从句中。
a.Metals expand when (they are) heated and contract when (they are) cooled . 金属具有热胀冷缩的特性。（两个when后省略they are）
b. When (he was) a student in the university, he read a lot.（when后省略了he was）
c. While (he was) waiting , he took out a magazine to read.
d.A tiger can't be tamed unless (it is) caught very young.
e._________, Dr. Smith has long been a severe critic of the govern-ment's economic policies.
A. Although not being an economist himself
B. Although not an economist himself
C. Although not having been an economist himself
D. Although being no an economist himself
正确答案：B。从句相当于：Although he is not an economist himself, ...
（2）从句没有be动词
如果状语从句中没有be动词的形式，我们则可以把从句主语省去并且把动词变成现在分词-ing形式。