/wen; US hwen ||; hwɛn/adv conj 1 at what time 什么时候: When did she arrive? 她什么时候到的？I don't know when she arrived. 我不知道她是什么时候到的。
2 used for talking about the time at which sth happens or happened （用于指事情发生的时间）: Sunday is the day when I can relax. 星期天是我可以轻松一下的日子。I last saw her in May, when she was in London. 我上次见她是在五月份，当时她在伦敦。He jumped up when the phone rang. 电话铃响时他跳了起来。
Notice that we use the present tense after ‘when’ if we are talking about a future time I'll call you when I'm ready. 指将来的事时，when之后用现在时：I'll call you when I'm ready.我准备好就打电话给你。 3 since; as; considering that 既然；考虑到: Why do you want more money when you've got enough already? 你的钱既然已经够用了，为什么还要更多的呢？ ☞ When is used for talking about something that you think or know will happen, but if is used for something you are not sure will happen. Compare I'll ask her when she comes (= you are sure that she will come). I'll ask her if she comes (= you are not sure whether she will come or not). when用于认为或知道行将发生的事，if用于不肯定是否会发生的事。比较：I'll ask her when she comes.她来这里时我问问她。·I'll ask her if she comes.如果她来，我会问问她。