If something happens when something else is happening, the two things are happening at the same time.
When eating a whole cooked fish, you should never turn it over to get at the flesh on the other side.
吃一整条鱼时,切忌把鱼翻过来吃另一边的肉。
You use when to introduce a clause in which you mention something that happens at some point during an activity, event, or situation.
When I met the Gills, I had been gardening for nearly ten years.
结识吉尔一家时,我已从事园艺工作近十年。
You use when to introduce a clause where you mention the circumstances under which the event in the main clause happened or will happen.
When he brought Imelda her drink she gave him a genuine, sweet smile of thanks.
当他给艾梅尔达拿来一杯饮料时,她以真诚、甜美的微笑表示感谢。
You use when after certain words, especially verbs and adjectives, to introduce a clause where you mention the time at which something happens.
I asked him when he'd be back to pick me up.
我问他什么时候回来接我。
You use when to introduce the reason for an opinion, comment, or question.
How can I love myself when I look like this?
我这个样子,怎么能自我欣赏呢?
You use when in order to introduce a fact or comment which makes the other part of the sentence rather surprising or unlikely.
Our mothers sat us down to read and paint, when all we really wanted to do was to make a mess.
我们的母亲让我们坐下读书、画画,而我们真正想要做的却是打打闹闹。
You use when to introduce a clause that specifies or refers to the time at which something happens.
He could remember a time when he had worked like that himself.
他还记得自己一度工作起来也是那个样子。
You use when to ask questions about the time at which things happen.
When are you going home?
你什么时候回家?
When did you get married?
你是什么时候结婚的?