If the Nightingales hadn't been built yet their beds had turned out to be needed, the outrage would rightly have been visible from space.

yet 除了用作副词外，还可以用作连词，等于 but，表示“转折”。

在 if 从句里面是两个并列的分句，yet 不是修饰  hadn't been built 的副词，不是状语；而是引导第二个并列分句 their beds had turned out to be needed。yet 之前通常有逗号停顿，但本句是一个 if 从句，为了紧凑而不用逗号。

又如：

I don't eat much, yet I am a size 16. 

我吃得不多，然而我是个穿16号服的人。

It was a rash thing to do, yet one cannot but admire her courage. 

这事做得过于鲁莽，但人们不能不佩服她的勇气。

She trained hard all year yet she still failed to reach her best form.

她全年艰苦训练，然而仍未达到自己的最佳状态。

当然， 作为副词的 yet，与过去完成时连用也没有任何问题。例如：

He had not yet gone to bed before I arrived.

He hadn't gone to bed yet before I arrived.