We can use the te form with the group 1 verb しまう to express regret at an action.

【Verb: te Form】しまう
Completely forget about an appointment with a friend.
I wasn’t thinking clearly at the time and I mindlessly said something horrible.

Note the te form is used with the present participle in the first clause.

I did the house chores and accidentally threw out something important.

The word “accidentally” isn’t explicitly used in the above sentence, but the use of しまう suitably expresses the nuance of regret so it is fair to assume that you didn’t mean to throw the item away.

If you don’t study hard while you’re a student you’ll come to regret it.
You’re not allowed to do that.

〜てしまう can only be used to express regret at an action has been or will be performed; it cannot be used with the negative te form to express regret at having not done something.

[I wish I had asked the girl I spoke to at the party for her phone number.]

Instead we would use the conditional with the past tense of good to say, “it would be good if I had done x”.

I wish I had asked the girl I spoke to at the party for her phone number.

This structure can also be used to express completion; however, this is a more minor usage. In the vast majority of cases when you hear ~てしまう it will be to express regret.

I was hungry and so finished three whole cakes.
To read three books on the airplane.

In casual conversation you will often hear an altered form of 〜てしまう.

  • Verbs with the te form ending 〜て ⇒ ちゃう
  • Verbs with the te form ending 〜で ⇒ じゃう
It seems I lost my wallet somewhere.
My daughter is hanging out with her friends every day.
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