I decided to paint the bathroom ceiling but ran out of steam halfway through.

分析：halfway through 中的 through 本来是介词，意思是“在……的过程中”，halfway through 的意思是“在……过程中的一半或中途”。如：

We left halfway through the film. 电影演到一半时我们就走了。

Halfway through the race the horse went lame. 那匹马赛跑中途跛了腿。

We stopped for refreshment halfway through the journey. 我们走到半路停下来，恢复一下体力。

但在语境明确的情况下，有时可省略 halfway through 后面的名词，此时 through 则成了副词。如：

It was a terrible film—I left halfway through. 这部电影糟糕得很，我看到一半就离场了。(可视为在 through 后面省略了 it 或 the film)

网友句子的句意：

I decided to paint the bathroom ceiling but ran out of steam halfway through. 我决定要粉刷浴室天花板，但干了一半就筋疲力尽了。