一、only if的用法
1、在only if 中，副词only是中心词,而从属连接词if只用来连接从句，因此，它表示“只有……(才);只有在……的时候，唯一的条件是……”的意思。
I'll do that, but only if we set a few rules.
我可以干，但我们得定几条规矩。
The placebo effect can be understood only if we acknowledge the unity of mind and body.
只有我们承认身心一体，安慰剂效果才能被理解。
The lawyer is paid only if he wins
只有官司赢了才支付律师费。
You can make progress only if you are modest.
惟虚心乃能进步。
Political attacks work only if they find an echo with voters.
政治攻击只有在选民中引起共鸣才会有作用。
The meat market can continue only if it is radically overhauled to conform with strict European standards.
肉类市场只有经过彻底的大检查，严格遵守欧洲标准，才能继续营业。
Syria will attend only if the negotiations deal with issues of substance.
只有在谈判涉及重大问题时叙利亚才会参加。
The two sides will clash there only if Chelsea beat Sunderland in their quarter-finalreplay.
只有切尔西队在四分之一决赛的重赛中击败桑德兰队，双方才会交锋。
Children may enter the club only if they are accompanied by adults.
儿童只有在成人陪同下才能进入俱乐部。
2、only if 有时也写成only…if，表示唯一的条件，但是意思不变。
I will only come home if you come with me.= I will come home only if you come with me.
我回家的唯一条件就是你跟我一道回家去。
3. 用于引导条件状语从句，比if所表达的语气更强烈。置句首可引起倒装。
Only if a teacher has given permission is a student allowed to leave the room. 只有老师允许学生才可以离开房间。
二、if only 的用法
1、在if only中，从属连接词if是中心词,而副词only只是来加强if的语气的，因此，if only仅是if的一种强化形式，而不仅能表达说话人的愿望。因此，它同if一样，也能引出条件状语从句（用陈述语气）。
If only he does his best, he will succeed.只要他竭尽全力，他就会成功。
2、if only常常用来表达强烈的愿望或遗憾，因此，主要用在虚拟语气中，用以表达强烈的愿望或非真实条件,一般指的是一个不可能实现的愿望或假设。常被译为“但愿、真希望”、“要是（若是）……该多好啊” “要是...就好了”等。
If only it were that simple!
要是那么简单就好了！
If only someone could help me!
有谁能帮助我就好了！
If only he could conceal his part in the accident
要是他能够隐瞒住自己在这场事故中的责任就好了。
If only he would hold her close to him.
要是他能将她拥入怀中就好了。
If only I could get some sleep, I would be able to cope.
要是能睡上一会儿，我就能应付了。
If only I could be master of this subject.
我多么希望能精通这门学科。
Oh! If only I'd known earlier.
嗳，早知道就好了。
If only you hadn't felt compelled to meddle.
要是你没有觉得非插手不可就好了。
Oh, if only he knew English!
唉，要是他懂英语该多好啊！
If only my father had possessed an ounce of business sense
要是我父亲有一点点经商的头脑就好了。
If only you had told me that some time ago
要是你早些时候告诉我该多好啊！
There had to be an answer — he was sure he could tease it out if only he had time.
一定会有答案——他肯定，只要他有时间就一定能弄个清楚。
If only the banks would lend money without limit!
但愿银行能无限制地贷款！
3、if only从句在大多数情况下用作虚拟条件句，但是偶尔也有用在真实条件句的情况。
与过去事实相反,后面跟过去完成时, had+过去分词
与现在事实相反,后面跟一般过去时.
与将来事实相反,后面跟一般过去时或should/would/might/could + 动词原形
例：
If only she would come!要是她能来就好了。
If only you had told me you had a spare ticket for the show. 如果你告诉我，你多了一张演出票就好了。
If only one had an unlimited supply of money!
If only one had an unlimited supply of money!
要是财源不断就好了!（与现在事实相反）
If only I could get some sleep, I would be able to cope.
要是能睡上一会儿，我就能应付了。
If only he arrived in time.
但愿他能及时赶到。
If only it would clear up soon!
但愿天气赶快放晴。
If only he comes back early.( 用在真实条件句的情况)
但愿他早点回来。
4. if only有时也可以写成if …… only，如：
If she would only come!但愿他能来。（要是他能来，该多好啊！）
5. if only 还可以用在不定式或介词短语前面，引出一个“虽然不是很好，但是总算还好”的理由，如：
I’ll have a glass myself if only to stop you from drinking it all.
我自己也喝一杯吧，哪怕只是为了不让你把它全喝下去。
If only for your own peace of mind you should know where the money came from.
哪怕只是为了你自己心绪宁静，你也应该知道这笔钱从何而来。
She writes me often, if only to scold me because I haven't written to her
她常来信，即使仅仅为了责怪我还没回信给她。
A one-to-one meeting with the US President was necessary, if only for a deeper exchange ofviews.
与美国总统一对一的会谈是必要的，即便仅仅是为了双方观点更深入的交流。
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