only if：只有、除非,意思相当于unless,但用于肯定句,引导条件状语从句：I will go only if you go with me.除非你跟我一起走,否则我就不走（只有你跟我一起走我才走）.
比较：unless,用于否定句：I won't go unless you go with me.
If only：只要,引导条件状语从句：If only it clears up,we'll go.只要天一放晴,我们就走.
要是……就好了,引导独立句：If only you had won the game!你要是赢了该多好啊.
Only if 用于句首，用倒装语序，Only if a teacher has given permission is a student allowed to enter this room. 只有得到老师的允许，学生才能进入这间教室。
if only： 但愿 表示对现在和将来的愿望，或与过去事实相反的愿望。 If only it would stop raining 真希望雨能停。
一、if only的用法。
if only后面跟的是虚拟语气的句子，表示与事实相反的假设。
If only I had more money, I could buy some new clothes. 要是我有更多的钱，我可以买些新衣服。
If only he had arrived in time. 要是他能及时赶到就好了。
If only I had known it, I wouldn't have troubled him. 要是我知道，我就不会麻烦他了。
If only that photograph weren't missing. 要是那张照片没丢失该多好。
二、only if的用法。
only if后面跟条件状语从句，用陈述语气，后面用部分倒装句，be动词、助动词和情态动词放在主语之前，表示一种充分非必要条件，意思是只要。
Only if you passed the entrance exam and became a college student, will I accept you as a friend.
只要你通过高考，成为大学生，我就会认你为朋友。
Only if we persist in carrying out the open door policy, will we achieve greater success in every field.
只要我们坚持改革开放，我们就能在各个方面取得更大的成就。