if only意思是”要是…就好了”，其后的谓语动词用虚拟语气。例如：

If only it would stop raining. 要是雨停了就好了。

If only i knew her address.要是我知道她的地址就好了。

If only you had warned him earlier.要是你早点警告他就好了。

If only i had taken his advice.要是我听了他的劝告就好了。