When, while等连词引起的从句省略结构

⑴When, while, unless, if, until, though等连词引起的从句可使用省略结构(通常用连词直接加上分词或其它成分)。能够省略需要具备以下条件之一：
①从句的主语和主句的主语一致，从句中同时含有系动词be(此时可省去主语和系动词be)
②从句的主语是it，谓语是系动词be(可省去it is/was)
⑵after和before引起从句时不能被省略，但它们可用作介词。
While _______ (cross) the street, he was hit by a car.
The research is so designed that once _____ (begin) nothing can be done to change it.
When _________ (compare) with the size of the whole earth, the biggest ocean does not seem big at all.
I won’t go to his birthday party, even if _______ (invite)
____ , so the boy recognized his mistake and corrected it.
A. After the teacher explained the situation
B. Having understood the situation
C. The teacher had explained the situation
D. After understanding the situation
You can enter the lab only after ____.
A. calling B. called
C. being called