aspect N4 essential casualpolitewritten

てしまう — completion and regret

てしまう

Meaning

① completion: finish ~ing / get ~ done completely

② regret / unintended: end up ~ing / ~ by mistake (regret or surprise)

① completion

この本ほんを一日いちにちで読よんでしまった。

I read this whole book in a day.

② regret / unintended

電車でんしゃに傘かさを忘わすれてしまった。

I went and left my umbrella on the train.

Attaches to	Form	Example
Verb (て-form)	V-て + しまう	言いう → 言いってしまう

ちゃう / じゃう — casual contraction: 食たべちゃう, 飲のんじゃう

① completion

宿題しゅくだいを全部ぜんぶやってしまおう。

Let's get all the homework done.

② regret / unintended

ケーキを全部ぜんぶ食たべてしまった。

I ended up eating the whole cake.

Which reading applies is told by context and tone — the same 食たべてしまった can mean 'finished eating it' (completion) or 'ate it all, oops' (regret).

Jengo shows てしまう the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.

Sources Compiled with reference to A Dictionary of Basic Japanese Grammar.