If it weren't raining so hard,we'd go to the town.中为什么用were

这是虚拟语气的用法.
表示与现在事实相反时,从句用过去时,主句用过去将来时.
If +主语+were/did/were doing,主语+should/could/might/would do
例如 If I were you,I should go at once.
虚拟语气的were除了在If I were you 的结构中不能改动外,其他有些情况可用was.但是一般用were来表示虚拟语气.