上海新東方夏令營，介紹
    Were they here now, they could help us.
    =If they were here now, they could help us.
    Had you come earlier, you would have met him
    =If you had come earlier, you would have met him.
    Should it rain, the crops would be saved.
    =Were it to rain, the crops would be saved.
    注意：
    在虛擬語氣的從句中，動詞'be'的過去時態(tài)一律用"were"，不用was， 即在從句中be用were代替。
    If I were you, I would go to look for him.
    如果我是你，就會去找他。
    If he were here, everything would be all right.
    如果他在這兒，一切都會好的。