一.only if是由充当前置润色语的副词only和从属连词if组合而成，only仅仅起突出浸染，并不改变if的词义（only if有时会分开，表示唯一的条件）：只有、除非,意思相称于unless,当用于肯定句,勾引条件状语从句：
The lawyer is paid only if he wins.
只有官司赢了才支付状师费。
Syria will attend only if the negotiations deal with issues of substance.
只有在会谈涉及重大问题时叙利亚才会参加。
Children are admitted only if accompanied by an adult.
儿童必须有成年人陪同方可入场。
I will go only if you go with me.
除非你跟我一起走,否则我就不走（只有你跟我一起走我才走）.
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浸染。
The meat market can continue only if it is radically overhauled to conform with strict European standards.
肉类市场只有经由彻底的大检讨，严格遵守欧洲标准，才能连续业务。
把稳：Only if 用于句首，用倒装语序:
Only if you pass the entrance exam and become a college student, will I accept you as a friend.
只要你通过高考，成为大学生，我就会认你为朋友。
Only if the red light comes on is there any danger to employees.
只有红灯闪亮时才有危及职工的险情。
Only if we persist in carrying out the open door policy, will we achieve greater success in every field.
只要我们坚持改革开放，我们就能在各个方面取得更大的造诣。
Only if a teacher has given permission is a student allowed to enter this room.
只有得到老师的许可，学生才能进入这间教室。
二.if only 仅仅是if的强化形式，不仅用在虚拟语气中，用以表达欲望或非真实条件；也可以用在真实条件句的环境。
用在虚拟语气是可以勾引单独句子；也可以勾引从句和主句一起利用。
1.当if only用在虚拟语气是可以勾引单独句子用法如下：
（1）表达过去已发生的事宜是不能或未曾实现的欲望，常含有后悔、惋惜的意思，此时谓语动词要用过去完成时：
If only you had told me that some time ago
假如你早些时候见告我该多好啊！
If only you hadn't felt compelled to meddle.
假如你没有以为非插手不可就好了。
If only the West had been more responsive to his peace overtures in the fifties.
假如西方在50年代能更积极地相应他的和平姿态就好了。
If only my father had possessed an ounce of business sense
假如我父亲有一点点做生意的头脑就好了。
Oh! If only I'd known earlier.
嗳，早知道就好了。
Look at the trouble I am in! If only I had followed your advice.
看我现在所处的困难，当初要采纳了你的建议就好了。（上海高考）
特例：用could have done来表达过去已发生的事宜是不能或未曾实现的欲望的虚拟语气：
If only I could have helped put matters right.
假如我当时能帮着把缺点纠正过来就好了。牛津高阶英汉双解词典
（2）表达与现在事实相反的假设，即表示遗憾、惋惜等。谓语动词常日用虚拟语气过去时：
If only Marx were still by my side to see this with his own eyes!
马克思本日如能同我站在一起亲眼看到这种情景，那该多好啊!
If only I were rich.
我假如富有该多好啊！
If only I knew her name.
我假如知道她的名字就好了。
If only it were that simple!
假如那么大略就好了！
If only I could swim!
我假如会拍浮就好了！
（3）表示与将来的事实可能相反的假设，谓语动词利用would/could+do;或虚拟语气过去时，例：
If only he would hold her close to him.
假如他能将她拥入怀中就好了。
If only I could get some sleep, I would be able to cope.
假如能睡上一下子，我就能搪塞了。
If only he could conceal his part in the accident
假如他能够遮盖住自己在这场事件中的任务就好了。
If only I could be master of this subject.
我多么希望能精通这门学科。
If only she could marry me!
她假如能嫁给我就好了。
If only someone could help me!
有谁能帮助我就好了！
If only it would clear up soon!
但愿景象赶紧放晴。
If only he arrived in time.
但愿他能及时赶到。
If only the banks would lend money without limit!
但愿银行能无限制地贷款！
2.当if only勾引从句和主句一起利用虚拟语气；分以下情形：
（1）. 表示遗憾：当我们对过去的某种情形感到遗憾时，可以利用"if only"来表达。例如：
If only I had studied harder, I would have passed the exam.
如果我当初学习更努力一些，我就会通过考试了。
If only I hadn't missed the train, I would have been there on time.
如果我没有错过火车，我就会定时到达那里了。
（2）. 表示欲望：当我们对现在的某种情形感到不满或对未来有某种期望时，可以利用"if only"来表达。例如：
If only I had more time, I could finish this project.
如果我现在有更多的韶光，我就能完成这个项目了。
If only it would stop raining, we could go for a walk.
如果雨能停下来，我们就可以去闲步了。
（3）. 表示非现实情形：当我们对某种不太可能发生的情形抱有抱负时，可以利用"if only"来表达。例如：
If only I could get some sleep, I would be able to cope.
假如能睡上一下子，我就能搪塞了。
If only I were taller, I could play basketball professionally.
如果我再高一些，我就可以成为职业篮球运动员了。
If only I had a million dollars, I would travel around the world.
如果我有一百万美金，我就会环游天下。
（4）. 与过去完成时连用：当利用"if only"表示对过去的遗憾时，常日会与过去完成时连用。例如：
If only I had known it, I wouldn't have troubled him.
假如我知道，我就不会麻烦他了。
If only I had told her the truth, everything would be different.
如果我当时见告了她原形，统统都会不同。
If only I hadn't lost my phone, I wouldn't be in such a mess now.
如果我没有丢失手机，我现在就不会陷入这样的困境了。
The pills might have helped him, if only he'd taken them regularly.
他当时假如按时服药，也容许以对他有帮助的。
If only Mr. Le had told me yesterday, I would have given him some help, but now it is too late.
如果李师长西席昨天见告我，我将会给他一些帮助。但是，现在为时已晚。
（5）. 与过去大略时连用：当利用"if only"表示对现在的遗憾或对未来的欲望时，常日会与过去大略时连用。例如：
If only I had more energy, I would exercise every day.
如果我现在有更多的精力，我就会每天磨炼身体。
If only I had more money, I could buy some new clothes.
假如我有更多的钱，我可以买些新衣服。
If only I had a car, I could drive to work instead of taking the bus.
如果我有一辆车，我就可以开车上班，而不用坐公交车了。
If only I could get some sleep, I would be able to cope.
假如能睡上一下子，我就能搪塞了。
（6）. 与过去进行时连用：当利用"if only"表示对过去的某种情形进行假设时，可以与过去完成进行时连用。例如：
If only I were studying at that time, I would have understood the concept better.
如果我当时在学习，我就能更好地理解这个观点了。
If only I had been paying attention, I wouldn't have missed the turning.
如果我当时把稳不雅观察，就不会错过转弯了。
3.当if only勾引从句和主句一起利用，不该用虚拟语气；也能勾引（真实）条件状语从句：
If only he gets this job, it will make a great deal of difference.
他假如能得到这个事情，情形就会大不相同。
If only we can get to the next petrol-station we’ll be all right.
假如我们能达到下一个加油站，我们就好办了。
If only I had known earlier，I'd have sent you a telegram.
我只要知道得早一点，就会拍电报给你了。
I’m sure he is up to the job if only he would give his mind to it.
只要他肯下定决心，我相信他能胜任这份事情。
If only it clears up, we'll go.
只要天一放晴,我们就走。
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，只要他有韶光就一定能弄个清楚。
If only she would listen to what I was saying we might be able to sort out this problem.
只假如她乐意听我说，我们大概就能够办理这个问题。
If only our horse can get ahead of the leading runner, it can win the race!
只要我们的马超过领先的赛手，它便能赢得胜利了！
