1.only if引导限制性条件状语从句用陈述语气，如主句在后面，则后面主句用部分倒装句，be动词、助动词和情态动词放在主语之前；表示一种充分非必要条件，表示“只有...(才)；只有在...的时候，唯一的条件是...的意思；与if引起的一般条件句相比，它有一种增强主句语势的作用。
经典例句：
He will succeed only if he does his best.=Only if he does his best will he succeed.
他只有尽力而为才能成功。
I'll come only if you really need me.
只有当你真正需要我的时候，我才来。
There's way to make life work that will work to your advantage only if you only think about it.
总有办法让生活变得,如你所愿，只要你愿意思考。
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。
I'll go only if you go with me.
除非你跟我去，我才去。
Only if you help others unselfishly, can you get others' help when you need it. 只有当你无私地帮助别人时，你才能在别人需要帮助的时候得到别人的帮助。
Only when they got to a safe place, could we regard ourselves as having done our duty. 只有当他们到达安全地方的时候，我们才能认为自己尽到了责任。
Only if you have interested in learning will you learn it well.
只有你对学习感兴趣，你才能学好它。
2．if only这个组合就是if的强化形式；它可以独立使用，用来表示由于客观条件限制，为某事不大可能或完全不可能实现而感到惋惜或遗憾的复杂心情，常用虚拟语气，表示与事实相反的假设。经典例句：
If only he had arrived in time.
要是他能及时赶到就好了。
If only that photograph weren't missing.
要是那张照片没丢失该多好。
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 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。
Oh! If only I'd known earlier.
嗳，早知道就好了。
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 be master of this subject.
我多么希望能精通这门学科。
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 he arrived in time.
但愿他能及时赶到。
If only she could marry me!
她要是能嫁给我就好了
If only you had told me that some time ago
要是你早些时候告诉我该多好啊！
If only I had more time to think about it.
但愿我有更多的时间考虑它。
If only I had followed your advice.
要是我接受你的建议就好了。
If only you would take my advice.
你如果听我的意见就好了。
