when
conj.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.
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".
Commonly used to introduce a subordinate clause; when the 'when' clause comes first, it is usually followed by a comma.
I will tell him when he will arrive.I will tell him when he arrives.In future-time clauses introduced by 'when', use the present simple tense instead of 'will'.