when
01
什么时候, 何时
used when we want to ask at what time something happens
例子
Can you let me know when the package arrives?
你能告诉我包裹什么时候到吗？
Can you tell me when the meeting begins?
你能告诉我会议什么时候开始吗？
02
何时, 当
used to refer to a specific situation or time
when
01
当, 在...的时候
used to indicate that two things happen at the same time or during something else
例子
He felt nervous when he had to give a presentation.
他当不得不做演示时感到紧张。
I always eat breakfast when I wake up in the morning.
我早上醒来时总是吃早餐。
02
当, 而
used to imply unexpected or ironic outcomes
例子
He laughed when others expected him to be angry.
当别人以为他会生气时，他笑了当。
She found hope when all seemed lost.
当一切似乎都失去时，她找到了希望。
03
当
used to provide a reason or explanation for the main clause
例子
I do n't understand why we need to rush when we still have plenty of time to finish the project.
我不明白为什么我们需要匆忙当我们还有很多时间来完成这个项目。
It does n't make sense to spend so much money on a new phone when my current one works perfectly fine.
当我现在的手机完全好用的时候，花这么多钱买一部新手机是没有意义的。
when
01
什么时候
used to represent the time when something happens as the subject or object of verb or preposition
例子
I do n't know when he'll arrive.
我不知道他什么时候会到。
I remember when we went to Paris.
我记得当我们去巴黎的时候。
