happen : 普通用词，泛指一切客观事物或情况的发生，强调动作的偶然性。
- No one knows exactly what happened but several people have been hurt.
- Anything could happen in the next half hour.
- A funny thing happened in the office today.
- I don't want to think about what might have happened if he'd been driving any faster.
occur : 较正式用词，可指意外地发生，也可指意料中的发生。
- An accident involving over ten vehicles has occurred in the east-bound lane.
- If any of these symptoms occur while you are taking the medication, consult your doctor immediately.
chance : 侧重事前无安排或无准备而发生的事，特指巧合。
- They chanced to be in the restaurant when I arrived.
- I chanced on/upon (= found unexpectedly) some old love letters in a drawer.
- Ten years after leaving school, we chanced on/upon (= unexpectedly met) each other in Regent Street.
take place : 多指通过人为安排的发生。
- The concert takes place next Thursday.