This grammar shows an action happened by accident. It also shows something regrettable or irreversible happened.
Verb [て-form] + しまうPrimary
Verb [て-form] + しまいます
This grammar shows an action happened by accident. It also shows something regrettable or irreversible happened.
大事な書類をなくしてしまった。
I've gone and lost the important documents.
この本は面白いから、今日中に全部読んでしまいます。
This book is so interesting that I'll finish reading all of it today.
まだ食べていないから、ケーキを全部食べてしまわないでください。
I haven't eaten yet, so please don't go and eat all the cake.
忘れてしまった宿題のことを先生に謝った。
I apologized to the teacher about the homework I had completely forgotten.
ダイエット中なのに、ついケーキを一個食べてしまいました。
Even though I'm on a diet, I couldn't help myself and ended up eating a piece of cake.
This grammar can show two feelings. It can mean you finished something completely. It can also mean you did something by mistake. You need to listen to how it is said. Or you need to read the other sentences. This helps you know which feeling it is.
This is different from "〜ちゃう" or "〜じゃう". Those are short, casual forms. You use them when talking with friends. "〜てしまう" is more formal. You can use it in writing or with people you do not know well.
Watch out: Many people get confused about 〜てしまう and 〜でしまう. The verb before it decides this. If the verb's te-form ends in 〜て, use 〜てしまう. If it ends in 〜で, use 〜でしまう. The "しまう" part is always the same.
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