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unscheduled + noun
The manager called an unscheduled meeting to discuss the problem.
经理临时召集了一次会议来讨论这个问题。
未列入日程的；不定期的
先理解核心含义，再看它在真实句子里的用法。
未列入日程的；不定期的
not scheduled; not on a regular schedule
The manager called an unscheduled meeting to discuss the problem.
经理临时召集了一次会议来讨论这个问题。
未安排的；即兴的。
Not scheduled; impromptu.
The train made an unscheduled stop because of a signal problem.
火车因信号问题临时停靠了一站。
We had to attend an unscheduled meeting this afternoon.
我们今天下午不得不参加一次临时会议。
The system needs unscheduled maintenance after the crash.
系统崩溃后需要进行计划外维护。
The manager called an unscheduled meeting to discuss the problem.
经理临时召集了一次会议来讨论这个问题。
unscheduled 表示“未列入日程的；计划外的”，不一定指“不定期”。
The flight was unscheduled, so we had no time to prepare.
这次航班是临时安排的，所以我们没有时间准备。
Not this
The meeting is unscheduled to begin at 9 a.m.
Use this
The meeting is not scheduled to begin at 9 a.m.
unscheduled 是形容词，意为“未列入日程的”，不能像过去分词 scheduled 那样接不定式 to do。表示“预定做某事但没有预定”时，须用 not scheduled to do。
Not this
The buses are very unscheduled this week.
Use this
The buses are very irregular this week.
unscheduled 指“不在日程表上的”，不表示“不规律的”，且通常不用 very 修饰。若要表示“发车时间不稳定、不正常”，应改用 irregular。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
计划外维护
The factory closed for unscheduled maintenance.
工厂因计划外维护而停工。
计划外停靠
The train made an unscheduled stop.
火车临时停站。
临时到访
She received an unscheduled visit from her boss.
她的上司临时到访。
计划外缺勤
His unscheduled absence worried the team.
他的计划外缺勤让团队很担心。
非计划停机
The server's unscheduled downtime caused delays.
服务器非计划停机造成了延误。
非定期航班
The airline added an unscheduled flight for the holiday.
航空公司为假期增加了一班非定期航班。
Use unscheduled naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
联想：un-表否定，scheduled是“已排入日程”，日历上没安排的日子就是不定期的。
语域：neutral 常见场景：travel timetables、maintenance schedules、visits and appointments 通常为中性用词，正式与非正式场合都能用；具体语气常随搭配不同，如 unscheduled maintenance 多用于书面通知，而 unscheduled visit 在日常对话中也不少见。
真正同韵词：scheduled（/ˈʃedjuːld/）和 rescheduled（/ˌriːˈʃedjuːld/）与 unscheduled 完全押韵。注意拼写相近但发音不同的词：schedule 和 reschedule 无词尾 /d/，module 拼作 -dule 但读作 /ˈmɒdjuːl/，mulled 拼作 -ulled 但读作 /mʌld/。
Dictionary facts from ECDICT, WIKTIONARY.