放在句首不是省略Be动词的原因,是因为此时形容词或者形容词短语作原因状语,跟非谓语动词作状语类似.如果不省略,那句子中岂不是有两个谓语动词?
例如：
She has finished her homework,so she watched TV.一般句
Having finished her homework,she watch TV.非谓语作状语
这虽然是非谓语的例子,但其实语法上有相似处,第二句中不是直接把has放句首,是因为第二句没有连词so,所以要转化成非谓语作状语的形式,所谓非谓语就不是谓语动词.同理,形容词作状语,是肯定不能带上be的,being还说得过去,但是英语语法中没有这样的用法,一般都是直接把形容词放在句首.
例如：
Eager to improve her English,she has to practice English every day.
等同于：As she is eager to improve her English,she had to practice English every day.