A 用 if+were而不用 if+was的情况
1 通常两种形式都可以用，were更适合较正式的英语：
If she was/were offered the job shed take it.
如果给她提供那份工作，她就会接受。(两者都可用。)
If Tom was/were here hed know what to do.
如果汤姆在这里，他会知道该怎么办。
2 If I were/was you I would/should是一种表示劝告的方式，were要比was用得普遍些：
If I were you I would wait a bit， he said.
他说：我要是你的话，就再等一会儿。
were用于带to的不定式结构的情况更为常见：
If Peter were/was to apply for the post hed get it.
要是彼得申请这个职务，他会得到的。
3 以助动词置于主语前而构成条件句时，则只用were而不用was：
Were I Tom I would refuse.
如果我是汤姆，我会拒绝的。
if 意为since时，was不能由were代替;在 if only和wish之后，were可以由was来代替。
B 在正式的英语中， if + 主语 + 助动词结构中可以省略if，同时使助动词位于主语前面：
If I were in his shoes相当于：
Were I in his shoes
如果我处于他的地位
If you should require anything 相当于：
Should you require anything
你需要什么的话
If he had known 相当于：
Had he known
要是他当时知道的话