一、省略连词if
有时可将条件从句的连词if省略，但此时应用倒装句型，即将从句中的were, should, had 等提到句首：
If I were at school again, I would study better.
= Were I at school again, I would study better.
如果我再上学，我会学习得更好。
If you had come earlier, you would catch the bus.
= Had you come earlier, you would catch the bus.
如果你早点来，你会赶上公共汽车的。
If it should rain tomorrow, we would not go climbing.
=Should it rain tomorrow, we would not go climbing.
如果明天下雨，我们就不去爬山了。
【注】① 若条件从句为否定句，否定词not应置于主语之后，而不能与were, should, had 等缩略成Weren’t, Shouldn’t, Hadn’t而置于句首。
可以说：Were it not the expense, I would go abroad now.
但不能说：Weren't it for the expense, I would go abroad now.
② 有时省略if后提前的had不是助动词：
Had he time, he would come. 假若他有时间，他会来的。(If he had time…)
摘自赵俊英《现代英语语法》P.1431.
二、省略条件句的主语和其后的动词be
若主从句主语一致，且谓语部分包含有动词be，通常可将主语和动词be省略：
If given the same treatment again, he is sure to get well.98上海高考题
如果再给他同样的治疗，他肯定会好起来的。（If he is given the same treatment, he…）
If given a little more time, Sophie have deciphered the message with no help from Langdon.《达芬奇密码》
如果再花一点时间，索菲本可以不用兰登的帮助就可以独立地破译密码的。
If repaired earlier, the tractor would not have broken down. 要是早点儿修一下，拖拉机就不会抛锚了。(=If it had been repaired earlier…)
在书面语体中上述句子的if是可以省略的。
三、省略“it（主语）+be”
If necessary, you can contact me at home.
必要的话，我在家时你也可以和我联系。
（If it’s necessary, you can…）
If necessary, I would send more farm-hands to help you. 如果需要的话我会派更多的民工去帮你。(If it was necessary, I would…)
If possible, let me know beforehand. 如果可能，可在事前通知我。
Send the goods now if ready.= Send the goods now if they are ready.
货物如已备好，请即送来。
If true, this will cause us a lot of trouble.= If it is true, this will cause us a lot of trouble.
这事若是事实，它将给我们造成许多麻烦。
He will come if asked.= He will come if he is asked.
他如被邀就会来。
The medicine is quite effective if taken in time.=The medicine is quite effective if it is taken in time.
这药要是能按时服用，效果是很好的。
If anyone, he knows. 如果有人知道，那就是他了。
There are few people nowadays, if any, who remember him.
当今记得他的人，如有的话，也不多了。
He seems to have little, if anything, to do with this.
若要说他和这事有什么相关的话，那也似乎是很少的。
If in doubt, ask your doctor. He can give you further information.
你若有疑问，可以问问医生. 他会向你作进一步的说明。
Their policies have changed little, if at all, since the last election.
自上次选举以来，他们的政策就算是有所变化，也变得很少。
If by any chance you can’t manage dinner tonight, perhaps we can at least have a drink together.
就算你今晚不吃晚饭，也许我们至少可以一起喝一杯。
She seldom, if ever, goes to the cinema. 她难得看电影。
He seldom if ever travels abroad. 他到国外旅行，即使有过，也是极少的。
I might see you tomorrow. If not, then it’ll be Saturday.
我可能明天去看你。如果不是明天，那就在周六。
Ask her if it is a convenient time. If not, can she suggest another possible time?
问问她那个时间方便不方便。要是不方便，那她可不可以提出一个可行的时间?
注：有时not还可修饰另一个词语。如：
If not today, tomorrow I’m sure you’ll get an answer.
如果今天得不到回信，明天准能得到。
This is one of the oldest buildings in town, if not the oldest.
这是城里最古老的房屋之一，如果不是最古老的话。
They must decide if such a plan can be implemented and if so, when.
他必须决定这样的计划是否能实施，而且要是能实施的话，又得决定何时实施。
注意以下if so与if not连用的情形：
He may be busy. If so, Ill call later. If not, can I see him now?
他可能忙，如是这样，我以后再来拜访。他如不忙，我现在可以见他吗?
if need be为习语，其含义相当于if it is necessary (如果有必要的话)。如：
I will come if need be. 如有必要我会来。
I’ll work at night if need be. 如果有必要我可以晚上工作。
If need be we can always bring another car. 如果有必要的话我们还可以再开一辆车来。
四、省略整个条件从句
这样的省略通常需要借助一定的上下文，即省略条件从句后，所剩下的主句的意思在一定的上下文中意思是清楚的：
I might see her personally. It would be better. 我可以亲自去看她，这样好一些。(=…If I saw her personally, it would be better.)
