Simple past tense and past participle of happen.(verb)
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Use "happened" in a sentence
"I swore under my breath, because it had happened without me even realizing it'the only way it ever happened_ 'anymore."
"I know I don’t know the truth about Andy or what happened, or why it happened but it happened."
"There we were again, left without a keyboard player, and I started asking around until somebody told me that I should go see this band that had this great singer and piano player, and his name happened to be Isaac Hayes."