核心问题
I'll help you if necessary.
请问 if 从句中隐含的主语是什么？
词典解答
在这里隐含的主语是整个主句。if necessary = if it (= I will help you) is necessary。 例如：
I had encouraged her to structure her hours and to work from home if necessary so that she could spend time with her children. (Hillary, Living History) 我鼓励她规划自己的时间，如有必要可在家里工作，这样她就能抽出时间来陪孩子。