This means an action has finished. It can show that something is completely done. It can also show you feel regret about it.
Verb [て-form] + しまったPrimary
This means an action has finished. It can show that something is completely done. It can also show you feel regret about it.
大事な書類をなくしてしまった。
I've gone and lost the important documents.
宿題はもう全部やってしまったから、遊びに行こう。
Since I've already finished all my homework, let's go play.
ダイエット中なのに、ケーキを全部食べてしまった。
Even though I'm on a diet, I went and ate the entire cake.
結局、彼は最後まで謝ってしまわなかった。
In the end, he didn't apologize (and that was the final, regrettable outcome).
つい、友達の秘密を話してしまった。
I accidentally ended up telling my friend's secret.
This phrase is like a chameleon. It changes its color based on the situation. It can mean you finished something completely. Or it can mean you did something by accident, and maybe you are sad about it. The way you say it, and what is happening, tells you which meaning it is.
This form is different from 〜ちゃった and 〜じゃった. Those forms are very casual. You use them with friends and family. 〜てしまった is more neutral. You can use it in formal writing.
Use this when you want to show an action is finished. It often sets up what happens next. It closes one part of a story. Then you can talk about new things.
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