~てしまう (regret/completion)
grammar239 sentences
The auxiliary ~てしまう attached to a verb te-form, marking either neutral completion ('finished completely / fully done, no undoing it') or emotive regret ('unfortunately X happened'). Context disambiguates the readings: 食べてしまった = 'ate it all up' (completion) or 'ate it [unfortunately]' (regret). Casual contractions: ~ちゃう / ~じゃう (past ~ちゃった / ~じゃった); masculine/dialectal ~ちまう / ~じまう (past ~ちまった / ~じまった).