when
conj.at the time that something happens. You use this to connect two events or to ask about a specific time.
conj.at or during the time that something occurs. Functions as a subordinating conjunction to introduce temporal clauses or as a relative adverb in certain syntactic structures.
“I will call you when I arrive at the station.”
“The lights usually flicker when the old elevator starts moving in the hallway.”
“When the final results were eventually published, the research team discovered that their initial hypothesis had been correct all along.”
Commonly used to introduce a subordinate clause; when the 'when' clause comes first, it is usually followed by a comma.
In future-time clauses introduced by 'when', use the present simple tense instead of 'will'.
From Middle English when(ne), whan(ne), from Old English hwonne, from Proto-West Germanic hwannē, from Proto-West Germanic hwan, from Proto-Germanic hwan (“at what time, when”), from Proto-Indo-European kʷís (interrogative base). Cognate with Scots whan (“when”), Dutch wanneer (“when”), wan (“when”) and wen (“when, if”), Low German wannehr (“when”), wann (“when”) and wenn (“if, when”), German wann (“when”) and wenn (“when, if”), Gothic 𐍈𐌰𐌽 (ƕan, “when, how”), Latin quandō (“when”). More at who. Interjection sense: a playful misunderstanding of "say when" (i.e. say something / speak up when you want me to stop) as "say [the word] when".
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.
For Mandarin speakers Mandarin lacks a distinct relative pronoun system, often using the particle 的 (de) to modify nouns. Learners frequently omit the relative 'when' in clauses, producing structures like 'I remember the time he arrived' instead of 'I remember the time when he arrived', or incorrectly substitute the interrogative word 什么时候 (shénme shíhou) in statements.
In Mandarin, 'when' is often expressed using the phrase ...的时候 (...de shíhou). This leads learners to redundantly use 'when' and 'time' together, resulting in errors like 'When the time I was young' instead of simply 'When I was young'.
For Japanese speakers Japanese has no native /we/ sequence — /w/ survives only before /a/ — so the onset is rebuilt as a /ɯ/+/e/ glide, written ウェ (we) in loanwords. 'when' /wɛn/ surfaces as /ɯen/, with the vowel raised to Japanese /e/.
Japanese uses the particle 'toki' (時) to mark time, which follows the verb; the sentence-initial position of the English conjunction results in word-order transfer errors where 'when' is placed at the end of the clause.