这个说法应该是错的.应该改为：We don't know if it will rain tomorrow.
if 既可以引导宾语从句,也可以引导条件状语从句.if引导宾语从句时,意为“是否”,从句的时态根据句子时间状语或主从句结构确定.而if引导条件状语从句时,意为“如果”,若主句为一般将来时或祈使句,则从句必须用一般现在时.
例子：
We don't know if it will rain tomorrow.
We won't go to the park if it rains tomorrow.