表示与现在、过去或将来事实相反/相违背时，或表示不现实的愿望的时候用虚拟语气。

如，If I were you ,I would wear a suit at the party.如果我是你，我会在晚会上穿西装。(与现在事实相反)

  I wish I were ten years youger. 我希望自己年轻十岁。