only if 表示"只有...才..."。当only if放在句首时，应该用倒装句型。所谓倒装句型，就是将句子中的"助动词"提前。
1. I told him he would succeed only if he tried hard.
我告诉他，只有努力才能成功。
2. Only if I get a job will I have enough money to go to school.
我只有找到了工作才有足够的钱上学。

if only 则表示"如果……就好了"。表示说话人的愿望之类的，通常是虚拟语气。用过去式，表示与现在事实相反；若用过去完成式，则表示与过去事实相反。
1. If only he had been here. 要是他当时在这儿就好了。
2. I dread the coming exams. If only I could have time to review my lessons. 我对即将到来的考试非常担心。如果我有时间复习该有多好啊。
3. If only I had another chance. 要是我再有一次机会就好了。
4. If only I had a rich father. 要是我有个有钱的爸爸就好了。