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

如果头痛只发生在晚上，那常常是因为缺少新鲜空气和氧气。

The crash occurred when the crew shut down the wrong engine.

机组人员关错发动机后飞机就坠毁了。

These snails do not occur on low-lying coral islands.

这些蜗牛不会出现在低洼的珊瑚岛上。

It did not occur to me to check my insurance policy.

我没想到要核对我的保险单。