/wɛn/
[adverb] Used to ask or talk about the time that something happens or happened. It can also be used to mean 'at the time that' or 'if and when' something occurs.
'When' is a cousin of 'who' and 'what' — all three descend from the same ancient root that simply meant 'which one.'
The word 'when' reaches back to the very foundations of the English language, growing from the Old English form 'hwonne' or 'hwænne,' which was used to ask about or refer to a point in time. This Old English form belonged to a large family of question words that all began with the sound 'hw-' — a family that included 'hwæt' (what), 'hwā' (who), 'hwær' (where), and 'hwȳ' (why). These words were not random companions; they were all built on the same Proto-Germanic root, '*hwa-,' which carried the general sense of 'which' or 'that one.'
Go further back, and the trail leads to Proto-Indo-European, the ancient reconstructed ancestor language spoken perhaps six thousand years ago across the Eurasian steppes. The root here is '*kʷo-' or '*kʷe-,' a demonstrative and interrogative particle used to point at things and people in a general, questioning way. This single syllable is one of the most productive seeds in all of language history — it gave rise not only to the English 'wh-' question words but also, through Latin, to words like 'qui' (who), 'quod' (what), and 'quando' (when), and through Greek to words like 'pote' (when) and 'pou' (where). The same ancient root even reached into Sanskrit as 'ka-,' meaning 'who' or 'which.'
In the shift from Old English to Middle English, 'hwonne' gradually lost its initial 'h' sound in everyday speech, becoming 'whan' and then settling into the familiar 'when' we use today. This loss of the 'hw-' pronunciation happened across almost all the 'wh-' words in English, though the spelling kept the 'wh-' as a kind of historical fossil. In some Scottish and Irish English dialects, the older pronunciation — sounding a breathy 'hw' before the vowel — survived much longer, and you can still hear it in certain regions today.
What is remarkable about 'when' is how stable its meaning has been. While many words have drifted dramatically from their origins — 'disaster' once meant a bad star, 'panic' came from a forest god — 'when' has always pointed at time. Its function as a connector between events ('when the sun sets, the stars appear') makes it one of the most essential threads in any sentence about cause, sequence, or condition. It does not just ask a question; it builds a bridge between one moment and another.
Interestingly, 'when' also carries a subtle conditional flavour that its siblings like 'where' and 'who' do not. Saying 'call me when you arrive' implies not just a moment in time but an event that is expected to happen — it edges toward the territory of 'if.' This double life as both a time word and a soft condition word makes 'when' unusually versatile, and it is one of the reasons the word appears so frequently in everyday speech, sitting comfortably among the hundred most common words in the entire English language.