were to do 表示对将来虚拟

If she were to be a teacher, she would be more independent. 表示“如果她（以后）想做个老师的话,那她现在就要独立点.”（例子不恰当敬请见谅）

is、am、are to do 表示现在计划去做某事（并且不知道动作是否会被实施）

If you are to be stronger,you should do more exercise.如果你（现在）想变得更强壮,那你就要多锻炼.（这也是可以的.）

if sb were sth 如果某人是某物,是虚拟语气,表示假设

因为“某人”不可能会是“某物”,因为始终是人,只能假设一下说成是“某物”.

If I were a bird, I could fly. "如果我（现在）是只鸟的话,我就可以飞了”

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虚拟语气中用were to do sth的难点

If I _____ that, I shouldn’t feel so helpless.

A. knew    B. were to know

答案是 A，但为什么不能是B呢？

在通常情况下，我们可以用 were / was to do sth 代替虚拟语气的一般过去时，如:

If I asked (=were / was to ask), would you help me? 如果我提出请求，你会帮助我吗?

If I asked (=were / was to ask) him, I’m sure he’d help us. 如果我请他，我相信他会帮助我们。

但如果从句谓语动词是表持续意义的状态动词，则通常不用were / was to do sth，而用过去式，你的句子中的 know 就是这样的动词：

If I knew that, I shouldn’t feel so helpless. 假如我知道那事的话，我就不会感到这么无助。（不能用 If I were to know that…

再如 be 也是一样：

If I were you, I’d have a talk with your parents. 如果我是你，我就会和你的父母谈谈。（不能用 If I were to be you…）