对于这个问题，有三种解释。
一、《柯林斯英语用法词典》（Collins COBUILD English Usage）没有从语法上解释，而是把这看作were本身的一个用法：在正式的语境中，were搭配I、he、she、it、there或单数名词，表达虚拟情境或不可能实际发生的事情。
词典原文摘录：Were has a special use in conditional clauses when these clauses are used to mention situations that do not exist, or events that are unlikely to happen. When the subject of the clause is I, he, she, it, there, or a singular noun, were is sometimes used instead of ‘was’, especially in formal writing.
但该词典也指出，在非正式语境中，也可以用was。
词典原文摘录：In conversation and in less formal writing, people usually use was.
词典例句：If I was an architect, I’d re-design this house.
同时该词典又指出，If I were you是固定搭配，即使在非正式语境中也不能用was。
词典原文摘录：The fixed phrase ‘If I were you’ almost always contains were, even in informal English. Don’t say ‘If I was you’.
二、某国外语法网站认为If I was会造成歧义，引起误解，该网站给出这样一个例句：
