if only/only if

only if 是由充当前置修饰语的副词 only 和从属连词 if 组合而成, 前者仅仅起突出作用, 并不改变 if 的词义。

only if 有时写作 only...if, 表示唯一的条件。例如:

1. I will only come home if you come with me. 只有你跟我一起走，我才回家。

2. I'll only come if you promise me that you won't invite Mary. 只有你答应不邀请玛丽,我才去。

3. “Will you come?” “Only if you promise not to invite Mary.” “你来吗?” “只要你答应不请玛丽。”

4. We can only derive information from students' errors if we are capable of describing them in an adequate manner. 我们只有对学生的错误作出适当的描述,才能从中取得信息。

5. He will only succeed if he does his best. 他只有竭尽全力才会取得成功。

6. ... ladies are only admitted to the library if accompanied by a Fellow of College or furnished with a letter of introduction. ……只要有学院评议员的陪同或持有介绍信，女士们才可以进入本图书馆。

7. These snakes only attack if they feel cornered or threatened. (EED) 这些蛇只有当它们感到陷入困境或受到威胁时才伤害人。

only if 位于句首时, 主句的主谓语要进行部分倒装, 见 only if $^{1}$ 。

if only 则应当看作是从属关联词,主要用在虚拟语气中,用以表达愿望或非真实条件:

1. If only she could have lived a little longer. (EED) 要是她能活得再长一些,那该多好!

2. If only he had arrived in time! 要是当时他能及时到达,该多好哇!

3. If only the rain would stop. 雨要是停了,就好了。

4. If only Marx were still by my side to see this with his own eyes! (Engels) 马克思今天如能同我站在一起亲眼看到这种情景,那该多好啊!

5. If only you hadn't made that noise they would never have known we were here. 如果当时你不出声的话,他们是绝不会知道我们在这里的。

6. If only she hadn't told the police, everything would have been all right. 要是当时她没有报告警察的话，那就什么问题也不会发生了。

7. If only I had more money, I could buy some new clothes. (PEU) 要是我的钱再多一些，我就能够买几件新衣服了。

if only 有时可以写成 if...only:

1. If I only knew! 要是我知道,就好了!

2. If I could only do it! 要是我能做这件工作,就好了!

if only 有时也能引导真实条件状语从句(见上条):

1. If only it clears up, we'll go. 只要天晴,我们就去。

2. He will succeed if only he does his best. 只要他竭尽全力,他就会成功。请做下面的英语试题:

3. Only if Albert had told me before last Friday, I would have given him some help, but now A B C it's too late. (EPT 教程)

4. The senior librarian at the circulation desk promised to get the book for me ____ she could remember who last borrowed it.

A. ever since B. much as C. even though D. if only(全国研考,96-3)