如果头痛只发生在晚上，缺少新鲜空气和氧气常常是其原因。

If headaches only occur at night, lack of fresh air and oxygen is often the cause.

《柯林斯英汉双解大词典》

它通常只发生在彼此非常亲近的人之间，比如夫妻。

It usually only happens between people who are really close to each other, like couples.

youdao

这两套不同的理论都意味着，适应不同的栖息地生活环境的这一过程只发生了一次。

Both of these scenarios imply that specialization to each niche occurred only once.

youdao

We know that two vehicles are going to come along and not help Tony and that the third vehicle will, because everything, as I say, happens in threes in folklore.

我们知道有两辆车会过来,而且它不会帮助托尼，那么第三辆就一定会，因为，我说过，在民间传说中什么都只发生三次。

耶鲁公开课 - 文学理论导论课程节选

That's a real fluid boundary between the divine and human realms, if you ask me. But it only happens there, in one spot.

这在神族和人类之间可真是一个不固定的界限，如果你问我的话，然而这种情况只在一个地点发生。

耶鲁公开课 - 旧约导论课程节选

So since that's the easiest scenario, you can probably be sure it's not going to be too frequently that you're just given the energy, right, that might be too easy.

所以既然那是最简单的情况，你可以确信只给你提供能量值的,这种情况不太可能发生太多，对，因为那太简单了。

麻省理工公开课 - 化学原理课程节选