省略主语：
当并列是成分是由同一个主语发出，并列连接词后可以省略主语：
She case forward and shook hands with me
省略谓语 ：
当连接词后面两个分句中的谓语动词相同时候，后一分局中谓语可以省略
I'd like to come but i can't
省略其他成分：
The boy wanted to bring his chest to the shopping center alone, but his mother told him not to
如果从句的主语和主句的主语一致，且从句的谓语含有be动词的某种形式，可同事省略从句的主语和be 动词的某种形式
when while 引导的时间状语从句：
Be confident of success when (you are) running the company
If , unless , once 引导的条件状语从句：
If (it is ) properly educated, child will become confident.
Once (you are) caught cheating in an exam, you will be punished
Though
参考
https://zhuanlan.zhihu.com/p/67310478