有一读者问：if only 与 only if 有什么区别？
二者都可看作短语从属连词，用以引导条件从句。if only 常用以引导一无结果主句的条件从句，表示愿望，相当于一感叹句，其谓语动词常用虚拟语气，如：
(1)If only there were forty-eight hours in every day.
(2)If only he could have lived a little longer.
if only 从句亦可用陈述语气，如：
(3)If only I haven't lost my watch!
if 与 only 亦可分开，如：
(4)If I only knew!（=If only I knew!）
(5)Oh, if I only had my life to live over again!
if only 有时则意为“只要”，如：
(6)If only somebody had told us, we would have warned you.
现在让我们谈谈 only if。这个短语连词常意为“只要”，实例如：
(7)He will take the job only if you give him a large salary.
(8)I could have lent him the bike only if he'd asked.
但请注意下面句子中 only if 的用法：
(9)You can come back. But only if you do what I say.（这句话的意思是：你不能回来，除非你听命于我。）
