Tomorrow will be rainy.We won't go climbing.(改为if引导的复合句)

问题描述:

Tomorrow will be rainy.We won't go climbing.(改为if引导的复合句)
→ We wouldn't go climbing if it was rainy tomorrow.
这是正确答案.
为什么不能改为:
We won't go climbing if it be rainy tomorrow.
为什么答案中用过去时那?

可以这样改