请问主语是什么, 为什么？分析句子成分

这是一个if条件从句，主句是How well you can control your body movement in the air.主句的句子成分：主语How well you can control your body movement in the air，系动词是is 表语是a decisive factor ，这是一个主系表结构，如果is换成是动词就是主谓宾结构。

主语是主语从句“How well you can control your body movement in the air ”，谓语动词是is，a decisive factor是表语， if you want to be a really good skateboarder是条件状语从句.
如果按照中文的语序，应该是，if you want to be a really good skateboarder，How well you can control your body movement in the air is a decisive factor.

How well you can control your body movement in the air是主语
这是一句主语从句
a decisive factor是表语（即主语补足语）
if you want to be a really good skateboarder是条件状语从句
从句本身一般是一句完整的句子（部分定语从句除外）

How well you can control your body movement in the air是主语，
IS是谓语
a decisive factor 是宾语，
if you want to be a really good skateboarder是句子的补语

How well you can control your body movement in the air