This means you finish doing something. You do it completely. There is nothing left to do.
Verb [て-form] + しまうPrimary
This means you finish doing something. You do it completely. There is nothing left to do.
夏休みが終わる前に、この分厚い本を全部読んでしまうつもりだ。
I plan to finish reading this entire thick book before summer vacation ends.
弟のケーキ、うっかり食べちゃった。
Oops, I accidentally ate my little brother's cake.
お客様に頼まれた資料は、昨日すべて作成してしまいました。
I finished preparing all the documents the client requested yesterday.
大事なパスワードを忘れてしまわないように、どこかにメモしておこう。
Let's write it down somewhere so I don't end up forgetting the important password.
彼は会社のお金をすべて使ってしまった。
He went and used up all of the company's money.
This grammar shows two feelings. It can mean you finished something completely. Or, it can mean something bad happened by accident. Think of it like a coin with two sides. The words around it tell you which side it is.
This is different from 〜ておく. Both use the 〜て form. But 〜てしまう means you finished something. It is done. 〜ておく means you do something to prepare. You do it for later. One is about finishing. The other is about getting ready.
Use this when you want to show you finished something completely. It means you did it all. It can also show a strong feeling about what happened.
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