Question

were在什么情况下主语可以是第一第三人称单数

Answer 1

在were的使用过程中，一般主语是第二人称，或者第一人称或第三人称复数，但是有一种例外情况，就是were用作非真实的过去式时，即用在假设的条件从句中，主语可以是第一人称第三人称 单数 。were用作非真实的过去式,表示虚拟的场景，即这种假设是不存在的。were用于虚拟的场景时,were比was更正式，更规范，不管你主语是单数还是复数。

例如：

(1)  If I were you. 如果我是你。(这种用法不管书面还是口头，都是这样，非常正式。If I were you已经成为了固定用法，不能用was)

(2)  As it were. 似乎，好像，仿佛。

(3)  If I were in his circumstances,I would do the same. 假如我在他的处境，我也会这么做。

(4)  If the law were changed,it would not benefit women. 假如法律作了修改，无益于保护女性。

(5)  If he were here，he would act differently. 假如他在场，他的表现会不一样。

(6)  If it were largely a matter of chance,two people would rarely agree on how to understand a line of poetry,and a poet would find it impossible to communicate,except in the most haphazard way,with his public.

如果这在很大程度上是一种随机事件，那么两个人很少会在如何理解一行诗的问题上达成一致，一位诗人将会发现，除了使用最随意的(haphazard)方式以外，要与他的读者(public)交流几乎是不可能的。

特别注意后面与would的搭配。

除了If I were you,As it were之外，其它的假设使用was或者were都是可以接受的，都不能说是错，但推荐使用were.

以上用法在<< COBUILD English Usage >>一书中有清晰的讲解。