1. 就你的句子表语 a long time 来看，只能用 is，不能用 are：

误：Eight years are a long time!

正：Eight years is a long time!

因为 a long time 的意思是“一段很长的时间”，是被作为整体来看待的。

2. Five years have passed since we last met.

你的上面这个句子用复数动词完全没有问题，它强调这五年是“一年一年过去的”，当然，如果把它看作一个整体，也可以用单数动词。就本句来说，用复数动词比用单数动词更合适。

赞同蒋老师的解答。

借此机会引用章振邦《新编英语语法概要》中的一段说明供网友参考：

如果数量概念被看作一个整体，谓语动词用单数；如果说话人侧重在一个个的个体，谓语动词就用复数。比如说，一个罪犯被监禁二十年，我们说“二十年监禁是他应得的惩罚”：

Twenty years in prison was the penalty he had to pay.

在这里，我们把twenty years看作整个的一段时间，意义上是单数，所以谓语动词用单数。如果我们把侧重点放在一个个的年份上，而说“自从他被送进监狱，二十年已经过去了”：

Twenty years have passed since he was sent to prison.

在这里，twenty years不论在语法形式上还是在意义上都是复数，因此，谓语动词用复数。

我的理解有些不同，不对的话，欢迎大家指正。

我觉得是否用单复数，如曾老师提到的章振邦语法书所讲的：“如果数量概念被看作一个整体，谓语动词用单数；如果说话人侧重在一个个的个体，谓语动词就用复数。”究竟用单用复，取决于作者的认知和习惯，和a long/short time, pass等词关系不大。请看外刊例句：

1. Ninety years are a long time in this new and vigorous country, and correspondingly when I think of the four - thousand - year history of Korea, it is a short time.

2. In terms of modern history five years are a long time.

3. Seven years are a short time, but these seven years have been very eventful.

4. The fact that nearly one hundred and fifty years has passed since Gluck wrote Orpheus...

5. Ten years has passed and the men, one quarter of them, have died.

6. Active storage space in the reservoir for the purposes for which the project is designed does not begin to be depleted until 50 years has passed.

因此，网友的句子Eight years is/are a long time! 都是正确的。

在这种情况下，谓语动词如果是be，通常用单数；如果是  pass，一般用复数。