The president, flanked by Secretary of State John Kerry and vice President Joe Biden, down played the claimed economic benefits of the pipeline, arguing it wouldn’t have lowered U.S. gas prices, created long-term jobs or reduced U.S. dependency on foreign energy.
wouldn’t have done 表示不会做某事，但是实际上做了。文章中的意思是本来不会降低美国的油气价格。
would have done
是一个固定虚拟语气的用法。
表示对过去事情的假设，意为“本来想做、要做但没做”，使用时大多会给出导致没做成或会这样做的条件。
英汉例句1：
If I had a chance，I would have set off on a journey round the world.
要是有机会，我早就环游世界了。（也可译为对现在的虚拟“要是有机会，我会环游世界”）
英汉例句2：
I would have told you the story about him, but you were not home.
我本来可以告诉你关于他的故事，但是你不在家。（由于：你不在家”这样的条件下，没有告诉关于他的的故事。）
5.would have done与if连用
例句：If I hadn’t been busy that day, I would have gone there with them.
如果那天我不忙，我会和他们一起去那儿。（事实上表示没有由于忙，而没有去。）
在这里也可以不用if
如：I was busy that day. Otherwise I would have gone there with them.
我那天很忙，否则，我就和他们一起去那儿了。
6.文章中lower 在这里作为动词，“降低”的意思。
