表示对将来情况的主观推测
从句：①if+主语+were to do （if 表示未来的虚拟语气独有该形式,其他如wish,even if等词都没有该形式来表示未来的虚拟语气）
②if+主语+should+do
③if+主语+过去式（be动词用were）
主句：主语+should/would+do
eg：
1．If he should come here tomorrow,I would talk to him.
如果他明天来这儿的话,我就跟他谈谈.（事实：来的可能性很小,不可能）
2．If there were a heavy snow next Sunday,we would not go skating.
如果下周日下大雪,我们就不能去滑冰了.（事实：下雪可能性很小,不可能）
3．If she were to be here next Monday,I would tell her about the matter.
如果她下周一来这儿的话,我就会告诉她这件事的始末.（事实：来的可能性很小,不可能）