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when

[hwen, wen; unstressed hwuh n, wuh n]
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adverb
  1. at what time or period? how long ago? how soon?: When are they to arrive? When did the Roman Empire exist?
  2. under what circumstances? upon what occasion?: When is a letter of condolence in order? When did you ever see such a crowd?
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conjunction
  1. at what time: to know when to be silent.
  2. at the time or in the event that: when we were young; when the noise stops.
  3. at any time; whenever: He is impatient when he is kept waiting.
  4. upon or after which; and then: We had just fallen asleep when the bell rang.
  5. while on the contrary; considering that; whereas: Why are you here when you should be in school?
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pronoun
  1. what time: Till when is the store open?
  2. which time: They left on Monday, since when we have heard nothing.
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noun
  1. the time of anything: the when and the where of an act.
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Origin of when

before 1000; Middle English when(ne), Old English hwenne; cognate with German wann when, wenn if, when (compare Gothic hwan when, how); akin to who, what
Can be confusedwhen whencewen when

when's

[hwenz, wenz]
  1. contraction of when is: When's the show over?
  2. contraction of when does: When's the next train leave?
  3. contraction of when has: When's he ever been an authority?
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Usage note

See contraction.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for when

when

adverb
    1. at what time? over what period?when is he due?
    2. (used in indirect questions)ask him when he's due
  2. say when to state when an action is to be stopped or begun, as when someone is pouring a drink
  3. (subordinating) at a time at which; at the time at which; just as; afterI found it easily when I started to look seriously
  4. althoughhe drives when he might walk
  5. considering the fact thathow did you pass the exam when you'd not worked for it?
  6. at which (time); over which (period)an age when men were men
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noun
  1. (usually plural) a question as to the time of some occurrence
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Word Origin

Old English hwanne, hwænne; related to Old High German hwanne, hwenne, Latin cum

usage

When should not be used loosely as a substitute for in which after a noun which does not refer to a period of time: paralysis is a condition in which (not when) parts of the body cannot be moved
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for when

adv.

Old English hwænne, hwenne, hwonne, from Germanic pronomial stem *khwa- (cf. Old Saxon hwan, Old Frisian hwenne, Middle Dutch wan, Old High German hwanne, German wann "when"), from PIE interrogative base *qwo- (see who).

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Online Etymology Dictionary, © 2010 Douglas Harper

Idioms and Phrases with when

when

In addition to the idioms beginning with when

  • when all's said and done
  • when in Rome do as the Romans do
  • when it comes to
  • when least expected
  • when one's back is turned
  • when one's ship comes in
  • when pigs fly
  • when the cat's away, the mice will play
  • when the chips are down
  • when the dust has settled
  • when the going gets tough, the tough get going
  • when the shit hits the fan

also see:

  • cross a bridge when one comes to it
  • push comes to shove, when
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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.