1. 发生
    When something occurs, it happens.

    e.g. If headaches only occur at night, lack of fresh air and oxygen is often the cause...
           如果只是在晚上头疼，那常常是由于缺乏新鲜空气和氧气的缘故。
    e.g. The crash occurred when the crew shut down the wrong engine...
           机务人员关错了引擎，随即飞机坠毁。

2. 存在；出现；有
    When something occurs in a particular place, it exists or is present there.

    e.g. The cattle disease occurs more or less anywhere in Africa where the fly occurs...
           在非洲凡是有这种蝇虫的地方，几乎都会出现这一牛病。
    e.g. These snails do not occur on low-lying coral islands or atolls.
           在地势低平的珊瑚岛或环礁上没有这种螺。

3. （想法或主意）被想到，被想起，浮现于脑海中
    If a thought or idea occurs to you, you suddenly think of it or realize it.

    e.g. It did not occur to me to check my insurance policy...
           我没有想到要核实我的保险单条款。
    e.g. It occurred to me that I could have the book sent to me...
           我想到可以让人把书寄给我。