(A)if only是“只要;要是;但愿”之意。if only作“只要”解时，重点在if，表示基本条件。if only作“要是;但愿”解时，其后的从句中用虚拟语气，表达说话人的愿望(或非真实条件)。例如：

1)If only you study hard,you’ll pass the exam.只要你努力学习，你考试一定会及格。

2)If only you don’t lose heart,you’ll find a way to overcomethe difficulty.只要不灰心，你就能找到克服困难的办法。

3)If only he were at home!他现在要是在家就好了![现在他没在家]

4)If only I had seen him!那时要是我见到他就好了![那时我没见他]

5)If only he arrives on time!但愿他按时到达![表示对将来的愿望]

(B)only if是“只有”之意，重点在only。例如：

1)Only if he does his best,he’ll succeed.(=He will onlysucceed if he does his best.)他只有竭尽全力才会取得成功。

1)Only if you promise him that you won’t invite Mary,he willcome.(=He’

only come if you promise me that you won’tinvite Mary.)只有你答应不邀请玛丽，他才会来。