从句省略主语和谓语的条件当主句和从句的主语一致时,省略从句主语和部分谓语.如果不一致,就需要用独立主格结构.1.when

从句省略主语和谓语的条件
当主句和从句的主语一致时,省略从句主语和部分谓语.如果不一致,就需要用独立主格结构.
1.when considering the drawbacks,perhaps pollution is of prime importance.这个句子的从句部分省略了主谓语:we are,但从句和主句的主语不一致啊,主干中的主语是pollution.那这里的从句为什么可以省略主谓语呢?
2.另外,当主句和从句主语不一致时,需要用独立主格结构,给举几个例,并且分析其逻辑主谓.
1.Be sure not to lose confidence when confronted with difficulties.这里的主语是隐藏了to yourself 即：Be sure not to lose confidence (to yourself) when confronted with difficulties.
2.After class was over,the students soon left the classroom.和The work done，we went home.这两个句子都是表述，课程结束后/工作完成后，为什么前句用的是一般过去时，后者用的是过去完成时呢？
3.独立主格结构改写正确？
I walking(when I was walking) in the street ,someone patted me on the shoulder.
I being ill(I had been ill),all my classmates came to see me.
There being no bus (there had been no bus),we had to walk home.
其实我不明白2、3两句，用的是being？但逗号后面的主句用的是过去时camehad，从时态来理解，改写时应该用过去完成时，但是为什么例子中不用been ill there been no bus?