it was because the heavy rain,we could dnt go out.

如果没有was,整个句子的谓语就没有了
it是形式主语（we could not go out是主语从句即真正的主语）,还有这句中间不应该有逗号
because the heavy rain在这里是错的,正确的应该为“because of the heavy rain”这个介词短语做表语,was是谓语
正确的句子为It was because of the heavy rain that we could not go out.