Question

If the weather had been better ,we could have had a picnic.But it(rained) all day 为什么用过去时态

If the weather had been better ,we could have had a picnic.But it(rained) all day 为什么用过去时态

Answer 1

因为这句话是对过去发生事情的虚拟。
与过去事实相反：条件从句，If I(we,you等)+had+过去分词，结果主句，I(we,you等)+would/could+have +过去分词.
所以If the weather had been better ,we could have had a picnic.这句话是对过去的虚拟.因此But it(rained) all day 用过去时，是过去的事情了。

Answer 2

If the weather had been better ,we could have had a picnic.
这是个表示与过去事实相反的虚拟语气的句子，意思和含义是：如果天气好些的话，我们就可以去野餐了（实际是，过去的时候天一直不好，所以我们野餐没有去成）
因此后面it rained all day，表示实际上过去一整天下雨。
或者用过去进行时也可以，it was raining all day. 整天都“在”下雨（强调在下雨）

Answer 3

这是虚拟语气。假设状语从句用过去完成时，主句用过去时 。表示与事实相反。

Answer 4

虚拟语气，假设的一个条件。