happen

Compared meaning: to happen or take place (verb).

比较义项：发生（动词）。

• occur

  More formal and written in register; typically used for accidents, incidents, or notable events; often used impersonally ('it occurs to me') or in formal reports.

  语域更正式、更书面；通常用于描述事故、意外或重大事件；也常用于非人称结构（如“it occurs to me”）或正式报告中。

  A rare chemical reaction can occur when the two substances are mixed together.

  当这两种物质混合在一起时，可能会发生一种罕见的化学反应。

• happen 本词

  More casual and everyday; used for ordinary events in conversation; commonly takes 'to' to say something affects a person, which occur does not do naturally.

  更口语、更日常；常用于谈论日常生活中的普通事件；常与介词 to 连用，表示某事发生在某人身上，而 occur 通常不这样自然搭配。

  I hope nothing bad happens to you while you're travelling alone.

  我希望你独自旅行时不会遇到什么不好的事情。

Both words mean that something takes place. Occur is more formal and written, often used for accidents or abstract events, with the event itself as the subject. Happen is more everyday and common, and often takes 'to' to show something affects a person.

两词都表示事情“发生”，但 occur 更正式、书面化，常用于描述意外、事故或抽象事件，主语通常是事件本身；happen 更口语、日常，使用范围更广，常带 to 表示“某人遭遇某事”。

Compared meaning: something happening or befalling someone (verb).

比较义项：（某事）发生，降临到某人身上（动词）。

• happen 本词

  Neutral, everyday word for any event occurring; works for good, bad, or trivial things; used constantly in speech and writing with no special tone.

  中性、日常用词，可用于描述任何发生的事情，无论好坏或大小事；口语和书面语中都极为常用，没有特殊语气。

  I don't know exactly when the accident happened.

  我不知道那场事故究竟是什么时候发生的。

• befall

  Formal, literary word almost always used for misfortune or tragedy happening to a person or place; rarely used in everyday conversation.

  书面、文学化用词，几乎只用于不幸或灾难降临到某人或某地身上；日常口语中很少使用。

  A terrible tragedy befell the family that winter.

  那年冬天，一场可怕的灾难降临到这家人身上。

Both words can mean something happens. Happen is plain and neutral, used for almost anything. Befall is literary and formal, used mainly for something bad happening to a person or place, and is rare in speech. This does not compare happen's sense of 'by chance'.

这两个词都可以表示事情“发生”，但 happen 是最普通、最中性的说法，几乎什么事都能用；befall 则专指降临到某人或某地的（通常是不好的）事情，书面语气很重，日常口语很少用。此处不比较 happen 的“恰巧”这一层含义（即副词性的 happen to do 结构）。