てしまう details

write + (and…) + end up doing

kaite shimau

end up doing; do completely

meaning: end up writing

大事だいじな書類しょるいを電車でんしゃに忘れわすれてしまう。

daiji na shorui o densha ni wasurete shimau.

I end up leaving an important document on the train. neutral

この漫画まんが、一晩ひとばんで全部ぜんぶ読よんじゃった。

kono manga, hitoban de zenbu yonjatta.

I read this whole manga in one night. casual

Examples

end up doing — unintended, regrettable, or past the point of undoing

大事だいじな書類しょるいを電車でんしゃに忘わすれてしまった。

daiji na shorui o densha ni wasureteshimatta.

I ended up leaving an important document on the train. neutral

ごめん、最後さいごのケーキ食たべちゃった。

gomen, saigo no keeki tabechatta.

Sorry, I ate the last piece of cake. casual

到着とうちゃくが遅おくれてしまい、申もうし訳わけございません。

touchaku ga okurete shimai, moushiwake gozaimasen.

I am terribly sorry that my arrival was delayed. formal

do completely — finish something off, down to the last bit

この漫画まんが、一晩ひとばんで全部ぜんぶ読よんじゃった。

kono manga, hitoban de zenbu yonjatta.

I read this whole manga in one night. casual

宿題しゅくだいは金曜日きんようびのうちにやってしまった。

shukudai wa kin'youbi no uchi ni yatte shimatta.

I got my homework completely done on Friday. neutral

ちゃう — てしまう→ちゃう — 食べちゃう. Conjugates as a godan verb (ちゃった, ちゃおう); the everyday spoken form.
じゃう — でしまう→じゃう — 飲んじゃう, 死んじゃう. Voiced counterpart of ちゃう.
ちまう — rough, traditionally masculine flavor — やっちまった. More common in fiction and older speech than in daily conversation.

In formal apologies the continuative てしまい、 hangs the clause open before 申し訳ありません — a shape worth memorizing whole.

終わる
読み終わった states the endpoint of a durative activity and nothing more; 読んでしまった adds totality or affect and also fits punctual verbs — 忘れてしまった works where ×忘れ終わった cannot.

ておく
Mirror-image stance on the same て slot — ておく does the action on purpose with a later payoff in view, てしまう registers it as done past recall, wanted or not; the contractions とく and ちゃう carry the same split through casual speech.