省略：
1)省略是为了避免重复，保持语言简洁的一种语法手段。尤其是在口语中，省略是个普遍现象，例如：
I have heard of the news. So have I.
He didnt go to the concert yesterday. Neither did I.
想一下，为什么以上句子的应答部分要采用倒装形式?
此外，此类省略形式的要注意的一个问题是：前后时态要一致。
2)就职称考试而言，大家要特别注意由when, while, whether, if, unless, although等引出的状语从句中的省略现象，比较：
不省略：The young boy was badly injured by a motorcycle when he was crossing the street.
省略：The young boy was badly injured by a motorcycle when crossing the street.
不省略：If irrigation is not well managed, it can be harmful.
省略：If not well managed, irrigation may be harmful.
从以上两个例子可以归纳出此类从句的省略基本规则：
只有当主句和从句的主语相同时才能采用省略形式，省略时将从句的主语连同be动词一起省略;
如果句子主语与从句的动词是主动关系，采用-ing形式;如果句子主语与从句的动词是被动关系，采用-ed形式;
根据以上规则，确定以下题目的正确答案：
If ______ a hot bath before sleep, a days tiredness will soon vanish.
A.taking
B.taken
C.to take
D.you take
解题思路：由于该句主句的主语是a days tiredness，不可能发出take a hot bath的行为，故不能采用省略，答案应为D.
在if / when / whether + it is necessary, possible, convenient, important, applicable 等结构时，则省去 it is, 只保留 necessary, possible, convenient, important, applicable等，这是习惯，例如：
Take the medicine when necessary.
注意：英语中一般不能说：you are necessary / convenient / possible/ important等，例如不能说： Take this medicine when you are necessary, 或Come when you are convenient.
3)what, when, whether, how等 + to do 的省略形式，例如：
You are a college student now;you should know what to do and what not to do.
The train leaves at midnight, but at present we have not decided whether to take the trip.
但是：why do, 中间不用 to 连接，例如：
Why not go and ask the teacher for help?