when / while 当……时

1) When: 主句的谓语动作正在进行，when 连接的从句的谓语动作才发生。

2) While: while 连接的从句的谓语动作正在进行，主句的谓语动作才发生。

He was doing some gardening __________ he heard the bell ring.

2. He heard the bell ring __________he was doing some gardening,

when

while