1.if only意思是“只要......，但愿”，属日常通俗体，单独用表示惊叹、愿望，常省略表示结论的主句。例如：

If only he could arrive in time!但愿他准时到达!《英语惯用法词典》

2.Only if a teacher has given permission is a student allowed to enter this room.出自《牛津词典》，句中permission前可用或者不用物主代词。only if意思是“只要能......”，表示只要实现某种条件，位于句首时，主句的谓语常用倒装。又如：

Only if the red light comes on is there any danger to employees. 只要红灯一亮, 就表示有危及职工的险情。《牛津高阶英汉双解词典》

▲ 其实，only+状语位于句首时，主句的谓语通常都要用倒装式。例如：

Only then did she tell him about the attack.直到那时她才把袭击的事告诉他。《朗文当代英语词典》

Only when a woman loves him despite his appearance will he regain his human form.只有当一个女人不顾他的外表而爱他时，他才会恢复人形。《美国当代英语语料库（COCA）》

3.同意网友深圳钟的看法，Only if you say the model is the best只是一个从句，后缺主句。