常见为“当……时候”在完型中有类似于“突然”的含义，例如I happened to be walking outside when it began to rain.

给出例句，我们做更精确的解答

同学你好，欢迎你到新东方问吧提问，你的问题解答如下：◙ adv. 1. (used in questions 用于问句) at what time; on what occasion• 什么时候；何时；什么情况下；什么场合下: »When did you last see him?  你上次什么时候见到他的？ »When can I see you?  我什么时候可以见你？ »When (= in what circumstances) would such a solution be possible?  什么情况下可以这么解决？ 2. used after an expression of time to mean 'at which' or 'on which'• (用于时间的表达方式之后)在那时,其时: »Sunday is the only day when I can relax.  星期日是我唯一可以休息的日子。 »There are times when I wonder why I do this job.  有时候我也不明白自己为什么要干这个工作。 3. at which time; on which occasion• 其时；当时；当场: »The last time I went to Scotland was in May, when the weather was beautiful.  我上次去苏格兰是在五月份,那时的天气好极了。 ◙ pron. • what / which time• 什么时候；何时: »Until when can you stay?  你可以待到什么时候？ »'I've got a new job.''Since when?'  "我有了份新工作。" "什么时候开始的？" ◙ conj. 1. at or during the time that• 在…时候；当…时；在…期间: »I loved history when I was at school.  我上学时喜欢历史。 2. after• 在…之后: »Call me when you've finished.  你完成后就打电话给我。 3. at any time that; whenever• 在任何…时候: »Can you spare five minutes when it's convenient?  方便时能占用你五分钟时间吗？ 4. just after which• 一…就；刚…就: »He had just drifted off to sleep when the phone rang.  他刚睡着电话铃就响了。 5. considering that• 考虑到；既然: »How can they expect to learn anything when they never listen?  既然他们从不听讲,他们怎么能指望学到东西呢？ 6. although• 虽然；然而；可是: »She claimed to be 18, when I know she's only 16.  她自称是 18 岁,可是我知道她才 16。 如果同学有意愿学习北美留学类考试，欢迎到北京新东方VIP北美项目学习，我们这里有1对1的老师给你提供专业的指导和建议。祝你考试顺利！--------------------------------------------------------------------------北京新东方VIP北美项目托福、SAT、ACT、AP、GRE等课程，权威名师、一对一点拨提分快！获取更多信息欢迎登陆北京新东方北美vip官网查询http://bj.xdf.cn/publish/portal24/tab8635/?s=dc，或拨打电话400-817-1818咨询。

同学，你好：adv.什么时候; （用于时间的表达方式之后）在那时; 其时; 当时;conj.在…时; 既然; 如果; pron.什么时候; 那时; n.时间，时候; 日期; 场合; 希望能够帮到你本文来自【上海新东方基础英语】，更多具体内容请访问【上海新东方】，转载请标明链接。

whenconj. 当...的时候ad. 何时, 什么时候pron. 什么时侯n. 时间adv.[疑问副词]什么时候, 何时[关系副词]当...时When will he come? 他什么时候来?It was a time when motorcars were rare. 那是汽车很罕见的时代。I still remember the night when Nanjing was liberated. 我还记得南京解放那天晚上的情景。收起更多词典展开更多词典 现代英汉词典when  adv.什么时候When can you come? 你什么时候能来？当…的时候at the time when we met 在我们见面的时候I lived in this village when I was a boy. 当我还是个孩子的时候我住在这个村庄里。词性变化conj.…的时候Edison was not a good student when he was a young boy. 爱迪生小时候不是一个成绩好的学生。When the plane arrived, some of the detectives were waiting inside the main building while others were waiting on the airfield. 当飞机抵达时,一部分侦探等在主楼里面,而另一部分则等在停机坪上。When I came home my wife was cooking dinner. 我回家时，妻子在做晚饭。如果；要是；既然I can't tell you when you won't listen. 既然你不想听，我就不告诉你了。其实He stopped trying, when he might have succeeded next time. 他不再试了，其实他可能下一次就成功。词性变化pron.什么时候When else shall we meet again, if August 1st is not convenient for you? 要是八月一日对你不方便,那么另外什么时候再碰头呢?He then asked when Mr Gilbert would be allowed to go home. 然后他又问,什么时候可以允许吉尔伯特先生回家。Since when have he had a car? 他从什么时候起开始有了一辆汽车？那时She wrote a month ago， since when we've heard nothing. 一个月前她来过信，从那时起我们就没听到过她的消息。The Ambassador's wife went on asking questions, when she suddenly noticed a big hole in her husband's hat. 大使的妻子继续问了好多问题,突然她发现丈夫帽子上有个洞。习惯用语hardly when 刚…就Hardly had I opened the door when he told me. 我刚开开门，他就告诉了我。(= scarcely when)[w??n] advat what time? over what period?.(used in indirect questions)at a time at which; at the time at which; just as; afterI found it easily when I started to look seriously althoughhe drives when he might walk considering the fact thathow did you pass the exam when you'd not worked for it? at which (time); over which (period)an age when men were men na question as to the time of some occurrence习惯用语say when to state when an action is to be stopped or begun, as when someone is pouring a drink柯林斯词典说明收起更多词典展开更多词典 WordNetwhenAdverb1. as soon as; "once we are home, we can rest"(synonym) onceWe agree to comply with the copyright notice and statements mentioned in WordNet license.

conj. 考虑到；既然；当…时adv. 什么时候，何时pron. 那时；什么时侯n. 时间