only if 意思“仅仅如果”，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。if only 用于虚拟语气，表示“要是……就好了”。它用来表达一个不可能实现的愿望或假设。
知识拓展与过去事实相反，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 + 动词原形。
例句辨析：
only if
1.Ill do that， but only if we set a few rules.
我可以干，但我们得定几条规矩。
2.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。
3.The lawyer is paid only if he wins
只有官司赢了才支付律师费。
4.You can make progress only if you are modest.
惟虚心乃能进步。
5.You will win the race only if you train hard.
只要你努力训练，你会赢得比赛。
if only
1.If only it were that simple!
要是那么简单就好了！
2.If only someone could help me!
有谁能帮助我就好了！
3.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。
4.If only I could be master of this subject.
我多么希望能精通这门学科。
5.if only I would driver the car.
要是我会开车就好了
