1.only if
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。
（1）only if中，only是副词修饰if，意为“只有……（才）”。
I will come home only if you come with me.除非你跟我一起走，我才回家。
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 可用作“only ...if ...”表示唯一的条件。
I will only come home if you come with me.除非你跟我一起走，我才回家。
The sale will only go ahead if the serial numbers match.
只有序号相符时，销售才能继续进行。
I'll do that, but only if we set a few rules. 我可以干，但我们得定几条规矩。
I'll tell you, but only if you don't tell anyone else.
我会告诉你，但前提是你不告诉别人。
Only if somebody can come up with a way to turn back the clock.
除非有人能想出办法让时光倒流。
Education works only if the recipients really want it.
教育只有在受教育者真正想要的时候才有效。
Only if one believes in something can one act purposefully.
只有当一个人相信某事时，他才会果断行动。
（3）only if置于句首时，主句用倒装语序。
Only if I get a job will I have enough money to go to school.
我只有找到工作才有足够的钱上学。
Only if we have enough money will we visit Europe next year.
只要有钱，明年我们就去欧洲。
2.if only引导虚拟语气，表达强烈的愿望，意为“要是……就好了”。 它用来表达一个不可能实现的愿望或假设。相当于I wish：
If only he would listen to me.
他要是听我的话就好了。（与现在事实相反）
If only I knew her name.
我要是知道她的名字就好了。（与现在事实相反）
If only I were as clever as you！ 要是我像你一样聪明该多好啊！（与现在事实相反）
If only I had known you then.
要是我那时候认识你就好了。（与过去事实相反）
If only I'd had one of them, it seems I could have had success, fulfilled my dreams. （与过去事实相反）
如果我有其中的一个，我似乎可能已经成功实现我的梦想。
If only they had attended our wedding anniversary.
要是他们参加我们的结婚纪念日就好了。（与过去事实相反）
If she would only come！ 但愿她能来。（与将来事实相反）
