[求助]关于“would have do/ to do/done”

关于“would have do/ to do/done”

这个句型，我每次遇到都措手不及，请各位NN帮忙解答。下有几个摘取的例句

Although most experts agree that such a tax would result in fewer Birangians buying new vehicles and gradually reduce the number of automobiles on Birangi’s roads, they contend that it would have little impact on Birangi’s air-quality problem.

没人解答，那我换一种问法，would have do/done表示虚拟，是说，将来可能有或过去应该有，但是都没发生，这样理解对不对？

然后would have to do是will have to do 的过去式，但为何说是客观事实，我就不理解了，该怎么用呢？

请各位NN帮忙解答，偶是真的不会拉