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。
occur 较正式用词，可指意外地发生，也可指意料中的发生。
chance 侧重事前无安排或无准备而发生的事，特指巧合。
take place 多指通过人为安排的发生。
参考例句
1. Please give it to my wife if anything should happen to me. 万一我遇到不幸，请把这个交给我妻子。
2. He watched to see what would happen. 他观察着，看会发生什么情况。
3. I shudder to think what might happen. 我想到可能发生的事而战栗。
英英解释
v.
1. come to pass
2. happen, occur, or be the case in the course of events or by chance
3. chance to be or do something, without intention or causation
4. come into being; become reality
5. come upon, as if by accident; meet with
