Occurred implies that something happened in the past. It focuses on the fact that something took place and is often used in a more formal context.
* Example: "The accident occurred at 3:00 PM."
Happen is more general and can refer to events in the past, present, or future. It emphasizes the element of chance or coincidence.
* Example: "What happened to you?" (past)
* Example: "It happens every day." (present)
* Example: "Something might happen." (future)
In summary:
* Occurred is used for past events and has a more formal tone.
* Happen is more general and can refer to any tense, focusing on chance.
You can often use them interchangeably, but sometimes one word will be more appropriate than the other depending on the context and desired tone.