英语虚拟语气中表示对将来情况的主观推测的那一类跟条件状语从句有什么区别吗?

If I were you,I'd go to the park.
如果我是你,但这是不可能的,所以用虚拟语气
If it rains,I'll go to the park.
If it rained,I'd go to the park.（现在正在下雨）谢谢你！那请问：If there were a heavy snow next Sunday，we would not go skating.这里there和would为什么不是 is 和will 呢？当然是可以用is和will的，一般虚拟语气肯定有语境，不会无缘无故的让你填虚拟如果发生的几率比较小这里是不是就要用were和would了？除非你已经相当确定下个周日下大雪才用虚拟应该是不怎么确定才用虚拟吧？一点确定，不然怎么会是假设呢还是用上面的例子：If I were you,I'd go to the park.因为我一定不可能是你，所以用了假设因为一定会下大雪，所以假设不下大雪用虚拟