Old English hwanne, hwenne; of Germanic origin; related to German wenn 'if', wann 'when'
The main uses of when are: question word:When did you iast see your father? relative pronoun:I cannot recall an occasion when he asked me for help before. subordinating conjunction introducing a noun clause:I haven’t told them when I start. subordinating conjunction introducing an adverbial clause of time:The off-season hotel had been empty when she arrived.