“Whenever” has two main functions. It can refer to repeated events: “Whenever I put the baby down for a nap the phone rings and wakes her up.” Or it can refer to events of whose date or time you are uncertain: “Whenever it was that I first wore my new cashmere sweater, I remember the baby spit up on it.”
In some dialects (notably in Northern Ireland and Texas) it is common to substitute “whenever” for “when” in statements about specific events occurring only once and whose date is known: “Whenever we got married, John was so nervous he dropped the ring down my décolletage.” This is nonstandard. If an event is unique and its date or time known, use “when.”
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