The fish(which)we bought were not fresh.此句中在we后为甚麽出现了bought和were两个过去式？

The fish which we bought作主语，bought的宾语是fish。
were是助动词。

从句可以分解为，
We bought the fish, the fish were not fresh.

为了更好理解，可以提出主干：
The fish were not fresh.

we bought 是修饰fish的，翻译成我们买的鱼。

实际上是个定语从句，省略了关系词which/that.

The fish(which)we bought were not fresh.
这是一句定语从句
(which) we bought [括弧内可以省略]
修饰该鱼:即 我们所买的

were是the fish的谓语