From
〜て+しまう
Always has a nuance of regret or unfortunate(ly). when used on its own, it implies messing something up
ああしまった oh crap!
Not with negative
It cannot be used with a negative form. ないてしまわない
Regrettably
can be used to denote that an act was performed unintentionally or regrettably. can only be done in retrospect according to daichi book
泣いてしまった。 I unintentionally cried.
時計が壊してしまいました I regrettably broke my watch.
Completely (...)
less common than the 'regrettably' use. works with any action verb. The nuance is "although possibly undesirable/unnecessary, finish/do it anyway".
小説を一日で読んでしまう。 Regrettably, I fully finished reading the novel in one day.
すぐ片づけてしまいますね。 I will have it fully tidied straight away.
夕食の前に宿題をやってしまおう。 その教科書は全部読んでしまった。
Abbreviation
Often abbreviated to 「〜ちゃう」(or in some cases 「じゃった」) incasual speech.
そろそろ遅くなっちゃうよ。 It's unfortunately already getting late.
魚はもう死んじゃった。 Unfortunately, the fish had already died.