if 之后常用 were ，而不是 was 。这种情况在正式和非正式文体中都很常见。在正式文体中，were 比 was 更为常用，许多人认为前者更为正确，尤其是在美国英语里。 were 的这种用法，在语法上叫做“虚拟语气”。

    If I were rich,I would spend all my time travelling.假如我很有钱，我会把全部时间用来旅行。

    If my nose were a little shorter, I'd be quite pretty.要是我的鼻子短一点，我就会相当漂亮。

    （以上摘自《牛津英语用法指南》第四版 239.4）

    就是说，在 if 引导的虚拟条件句中，如果谓语动词是 be , 不管主语是什么人称，在正式和非正式文体中都常用 were，而不是 was 。

    如果主语是 I 或者单数第三人称，“也可以用 if ... was ...,但有人认为这种用法不正确”（以上《牛津英语用法指南》第四版 240.1）

    在虚拟条件句中，当主语是第一人称（I）或第三人称单数 (he,she,it)时，通常用 were，也可用 was，但在正式场合要用 were ，在 If I were you 中要用 were；省略 if 句子倒装时，只能用 were ； were 的否定式可用 were not 或 weren't。例如：

    If I were you, I would go with him. 如果我是你，我就同他一起去。

    If Peter were / was here, he would scold you. 如果彼得在这里，他会责骂你的。

    Were I to do it, I would do it in a different way. 要是让我做这件事，我会用不同的方式做。

    （摘自徐广联主编《大学英语语法讲座与测试》第五版p.709）

    所以，在网友的句子,"If it wasn't for the mist we could see your home across the bay," said Gatsby.中If it wasn't for the mist we could see your home across the bay,属于非正式用法，只能用于口语，在正式语体中，即在正式场合

应该用 If it weren't ...