ago和before的用法区别是什么？

ago和before都有“以前”的意思，那它们的用法区别是什么呢？
I.ago.“…以前”指从此刻起，若干时间以前，通常与过去连用。如：
①It happened two days ago..这件事发生在两天以前。
②I met him a few minutes ago.我在几分钟以前碰到他。
Ⅱ.before adv,prep&conj“。…以前”指从那时起若干时间以前。通常与完成时、过去时等连用。还可用作前置词或连接词表时间，而ago则不能这样用。
1. He said that he had seen her two days before.他说他两天前见到过她。（表从她说话那时起两天前)
2. I had been fine the day before.(那天)前一天的天气很好。
3. I've seen that film before.
4. I never met him before.