是改成虚拟
改：
We should have allowed the oil company to build this paper mill.therefore,we would not have water pollution now.
前面一句是对过去的虚拟,用should have done；后面一句是对现在的虚拟,用would do.should和would的选择是根据句意的,前句是“应该”,后句是“能够”.