状语从句主谓成分的省略，须遵循下面原则：
当从句的主语与主句的主语一致时，从句可以省略主语，从句的谓语动词采用其V-ing结构。例如：
Although working very hard, he failed in the final exam. (Although he worked very hard）
After taking the medicine, she felt much better. (After she took the medicine)
If going there by air, we’ll have to pay twice the fare. (If we go there by air)
当从句的主语和主句的主语一致时，若从句的谓语动词是系动词be或包含be的某种形式，此时从句的主语和be动词通常可以一同省略
- Whenever (she is) free, she often goes shopping.她有空就去逛商店。
- Work hard when (you are) young, or you’ll regret.少壮不努力,老大徒伤悲。
- While (he was) a young boy, he was always ready to help others. 他在孩提时代就乐于助人。
若从句的主语和主句的主语不一致，但和主句的宾语一致时，从句中的主语和be动词亦常可省略。例如：
- Father advised me not to say anything until (I was) asked.
当从句的主语是it, 且谓语动词是be或包含be的某种形式时, it和be常可同时省略。例如:
If (it is) possible, he will help you out of the difficulty.如果可能的话,他会帮你摆脱困境。
You must attend the meeting unless (it is) inconvenient to you.除非情况对你来说不方便,否则你必须出席这次会议。
