一.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，only仅仅起突出作用，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,意思相当于unless,当用于肯定句,引导条件状语从句：
The is paid only if he wins.
只有官司赢了才支付律师费。
Syria will only if the deal with of .
只有在谈判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）.
work only if they find an echo with .
*治攻击只有在选民中引起共鸣才会有作用。
The meat can only if it is to with .
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。
注意：Only if 用于句首，用倒装语序:
Only if you pass the exam and a , will I you as a .
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 in out the open door , will we in every field.
只要我们坚持改*开放，我们就能在各个方面取得更大的成就。
Only if a has given is a to enter this room.
只有得到老师的允许，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，用以表达愿望或非真实条件；也可以用在真实条件句的情形。
用在虚拟语气是可以引导单独句子；也可以引导从句和主句一起使用。
1.当if only用在虚拟语气是可以引导单独句子用法如下：
（1）表达过去已发生的事件是不能或未曾实现的愿望，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
要是你早些时候告诉我该多好啊！
If only you hadn felt compelled to meddle.
要是你没有觉得非插手不可就好了。
If only the West had been more to his peace in the .
要是西方在50年代能更积极地响应他的和平姿态就好了。
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。
Oh! If only Id known .
嗳，早知道就好了。
Look at the I am in! If only I had your .
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件是不能或未曾实现的愿望的虚拟语气：
If only I could have helped put matters right.
要是我当时能帮着把错误纠正过来就好了。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。谓语动词通常用虚拟语气过去时：
If only Marx were still by my side to see this with his own eyes!
**今天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.
我要是富有该多好啊！
If only I knew her name.
我要是知道她的名字就好了。
If only it were that !
要是那么简单就好了！
If only I could swim!
我要是会游泳就好了！
（3）表示与将来的事实可能相反的假设，谓语动词使用would/could+do;或虚拟语气过去时，例：
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 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 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!
她要是能嫁给我就好了。
If only someone could help me!
有谁能帮助我就好了！
If only it would clear up soon!
但愿天气赶快放晴。
If only he arrived in time.
但愿他能及时赶到。
If only the banks would lend money limit!
但愿银行能无限制地贷款！
2.当if only引导从句和主句一起使用虚拟语气；分以下情况：
（1）. 表示遗憾：当我们对过去的某种情况感到遗憾时，可以使用"if only"来表达。例如：
If only I had , I would have 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 the train, I would have been there on time.
如果我没有错过火车，我就会准时到达那里了。
（2）. 表示愿望：当我们对现在的某种情况感到不满或对未来有某种期望时，可以使用"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时间，我就能完成这个项目了。
If only it would stop , we could go for a walk.
如果雨能停下来，我们就可以去散步了。
（3）. 表示非现实情况：当我们对某种不太可能发生的情况抱有幻想时，可以使用"if only"来表达。例如：
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 were , I could play .
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 , I would the world.
如果我有一百万美金，我就会环游世界。
（4）. 与过去完成时连用：当使用"if only"表示对过去的遗憾时，通常会与过去完成时连用。例如：
If only I had known it, I have him.
要是我知道，我就不会麻烦他了。
If only I had told her the truth, would be .
如果我当时告诉了她真相，一切都会不同。
If only I hadn lost my phone, I 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 hed taken them regularly.
他当时要是按时服药，也许可以对他有帮助的。
If only Mr. Le had told me , I would have given him some help, but now it is too late.
如果李先生昨天告诉我，我将会给他一些帮助。但是，现在为时已晚。
（5）. 与过去简单时连用：当使用"if only"表示对现在的遗憾或对未来的愿望时，通常会与过去简单时连用。例如：
If only I had more , I would every day.
如果我现在有更多的精力，我就会每天锻炼身体。
If only I had more money, I could buy some new .
要是我有更多的钱，我可以买些新衣服。
If only I had a car, I could drive to work of 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.
要是能睡上一会儿，我就能应付了。
（6）. 与过去进行时连用：当使用"if only"表示对过去的某种情况进行假设时，可以与过去完成进行时连用。例如：
If only I were at that time, I would have the .
如果我当时在学习，我就能更好地理解这个概念了。
If only I had been , I have the .
如果我当时注意观察，就不会错过转弯了。
3.当if only引导从句和主句一起使用，不使用虚拟语气；也能引导（真实）条件状语从句：
If only he gets this job, it will make a great deal of .
他要是能得到这个工作，情况就会大不相同。
If only we can get to the next we’ll be all right.
要是我们能达到下一个加油站，我们就好办了。
If only I had known ，Id have sent you a .
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工作。
If only it up, well go.
只要天一放晴,我们就走。
There had to be an — 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 she would to what I was we might be able to sort out this .
只要是她愿意听我说，我们也许就能够解决这个问题。
If only our horse can get ahead of the , 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！
