I had been puzzling over the problem for over an hour without any result, when suddenly solution flashed across my mind.

这个问题把我难住了一个多小时，后来我突然开窍了，找到了答案。(时间状语从句)。

youdao

I had been puzzling over the problem for over an hour without any result, when suddenly solution flashed across my mind.

这个问题把我难住了一个多小时，后来我突然开窍了，找到了答案。(时间状语从句)。

youdao