While · 从句常放「背景带」 While we were playing football, it started to rain heavily. While 引导的那半句，多描述较长、可延续的动作或状态。
When · 可接「点事件」 We were driving home when the strong winds started. when 后常跟突然发生、瞬间开始的短动作（started / rang / found…）。