你这个问题可以阅读这个解答：

Well, the site was close to the Murgab river, which they would have depended on for their water.

【问题】请问这里为什么要使用过去将来完成时？

【翻译】那地方紧靠穆尔加贝河，他们当时或许要靠这条河用水。

【讲解】would have depended 表示对过去情况的可能性推测，意为“那时或许依靠”。

would +完成时，可以表达两种情况：

1. 表示虚拟语气，与过去情况不符或相反。例如：

If we had had enough rain last year, we would have gained a good harvest. 如果去年雨水充足的话，我们会有一个好的收成。

这句话的含义是：去年雨水不足，我们粮食没有取得好收成。

She would have gone with me, but she didn't have time. 她本来会和我一起去的，可她没有时间。

2. 表示对过去发生事情的可能性推测。例如：

People are recycling many things which they would have thrown away in the past. 现在人们都在回收他们过去可能会丢弃的东西。（2014安徽高考试题）

你的句子就属这种情况。

▲一般说来，情态动词would, might, could, should 都可以跟完成时连用，表示：1. 【与过去不符的虚拟语气】和2.【对过去的推测】

might have done：

【虚拟语气】We might have arrived earlier if we had taken the other road. 如果我们当时走了另一条路，就可能到得早一些。

【对过去的推测】You were stupid to try climbing up there. You might have killed yourself. 你从那儿往上爬，太蠢了，你可能会摔死的。

could have done：

【虚拟语气】If he had known the facts, he could have told us what to do. 如果他了解事实，他可能就告诉我们怎样做的。

【对过去的推测】Tom could have taken the money; he was here alone yesterday. 有可能是汤姆把钱拿走了，昨天他单独一个人在这里呆过。

注：could 表推测时，多用于否定句或疑问句。

should have done:

【虚拟语气】（分两种情况）

①I should have answered the letter yesterday, but it went right out of my mind. 我本该昨天回信的，但我把这件事忘得一干二净。（应该做而没做）

②But for the storm, we should have arrived earlier. 要是没有碰到暴雨，我们还会早到一些的。（用于第一人称的虚拟语气，跟would同）

【对过去的推测】

The train should have already left by then. 那时火车大概已经开走了吧。