时间状语从句的理解
Seeing a bear coming,he fled.
The teacher coming,the students inmediately became quiet.
在语法书中第一个句子逗号前称为Participle Phrase,第二个逗号前是Absolute Construction有什么区别呢（一个有主语,一个无主语?）
另外两个句子能否理解为是时间状语从句的简写呢：
When he saw a bear coming,he fled.
When the teacher came,the students immediately became quiet.