“only if” 和 “only ” 倒装有什么区别？

only位于句首有三种状况会引起句子倒装
1.only+副词 
例句：Only then did I realize the importance of study.
2.only +介词短语
本题就是这种状况
3.Only +状语从句（主句倒装,从句不倒装）
例句：Only When I graduated from school,did I realize the importance of study.
但要注意：only +名词或代词作主语,句子不倒装
例句：Only he knows the answer to the question.
only if 是只要的意思,句子不需要倒装,因此本题中的B不正确

only if 和 if only 的区别：

only if表示"只有"；if only则表示"如果……就好了".If only也可用于陈述语气.
I wake up only if the alarm clock rings.只有闹钟响了,我才会醒.
If only the alarm clock had rung.当时闹钟响了,就好了.
If only he comes early.但愿他早点回来.

辨析only if 与 if only 
only与if这两个单词随意组合,可以组成两个意义相近,但用法不尽相同的短语only if与if only.现将它们的具体用法辨析如下：

1. only if意为“only on the condition that,as long as”,“只要；它用于陈述事实或必要的条件,比if所表达的语气更强烈。

   You'll pass this course only if you study.只要你学,你就会通过这门课.

   Children may enter the club only if they are accompanied by adults.只要孩子们有大人陪同就可以参加俱乐部.

   The students at the university can take up part-time job only if it doesn't interfere with their studies.只要不影响学业,这所大学的学生可以从事兼职工作.

2. if only则有两种含义.第一种含义与only if近似,意为“as long as”,“只要”.

   例如：
   If only it clears up,we'll go.只要天一放晴,我们就去.

   第二种则是“expressing a wish or assumption”,“但愿；如果……就好了”.它用来表达一个不可能实现的愿望或假设,句子要用虚拟语气.

   例如：
   If only she would come!要是她能来就好了!
   If only you had told me you had a spare ticket for the show.如果你告诉我,你多了一张演出票就好了.
   If only you had worked with greater care!你要是工作更仔细一些该多好!