一、if only可表示对现时或未来的愿望,也可表示与过去事实相反的愿望,作“只要…但愿,要是…就好了”解。
第一种用法：相当于as long as 只要，连接2个句子，从句是条件状语从句：
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 it clears up,we’ll go.只要天一放晴，我们就去。
第二种用法：要是……就好了；如果……就好了；但愿
用来表达一个不可能实现的愿望或假设，句子多用虚拟语气，偶尔有时也用陈述语气：If only she would come!要是她能来就好了。
If only she come early.要是她早点来就好了。
If only I knew then what I know now.
要是当初我就懂了我到现在才领悟的东西，该多好啊！
其虚拟语气用法如下：
（一）表示与现在事实相反的假设，用一般过去时：
If only she could marry me! 她要是能嫁给我就好了。
If only I were as clever as you.要是我像你一样聪明该有多好啊！
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。
If only I knew her name.
我要是知道她的名字就好了。
If only I were rich.
但愿我很富有。
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。
《柯林斯高阶英汉双解学习词典》
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。
If only it were that simple!
要是那么简单就好了！
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 I could be master of this subject.
我多么希望能精通这门学科。
If only someone could help me!
有谁能帮助我就好了！
If only he arrived in time.
但愿他能及时赶到。
Oh, if only he knew English!
唉，要是他懂英语该多好啊！
（二）表示与过去事实相反的假设，用过去完成时：
If only we had arrived in time,we would,we would not have missed the train. 如果我们及时到达，我们就不会错过火车了。
If only he had been here.要是他当时在这儿就好了。
The pills might have helped him, if only he'd taken them regularly.
他当时要是按时服药，也许可以对他有帮助的。
《牛津高阶英汉双解词典》
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 somebody had told us, we could have warned you.
如果有人告诉了我们,我们就能事先提醒你。
If only I had another chance.要是我再有一次机会就好了。
Look at the trouble I am in! If only I had followed your advice.
看看我现在的困境，要是我当时采纳你的建议就好了。
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。
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 I'd known earlier.
嗳，早知道就好了。
摘自《汉英大词典》
（三）表示与将来事实相反的假设，用would +动词原形
If only it would stop raining.但愿雨快停下来。
If only he would hold her close to him.
要是他能将她拥入怀中就好了。
If only the banks would lend money without limit!
但愿银行能无限制地贷款！
摘自《柯林斯高阶英汉双解学习词典》
If only it would clear up soon!
但愿天气赶快放晴。
二、only if 表示：唯一的条件是，只有在……的条件下（的时候）才
放在句首，句子倒装：
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。《牛津高阶英汉双解词典》
Children are admitted only if accompanied by an adult.
儿童必须有成年人陪同方可入场。
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 lawyer is paid only if he wins
只有官司赢了才支付律师费。
《柯林斯高阶英汉双解学习词典》
Use a fire extinguisher only if you have been trained to do so ．
只有经过训练后才使用灭火器。高考真题- 2018 天津
So, everyone can have a good fortune only if he tries his best.
所以，每个人只有拼尽全力才能获得好运。
中考真题- 2018 衡阳
Where the penalties are not a must, they should be imposed only if the facts of a case support it.
非必须的处罚只有在案件事实支持的情况下才应予以实施。
四级真题- 2015年