auxiliary N3 common casualwritten

てやる — do ~ for (an inferior/animal)

てやる

Builds on て形

Meaning

① ①: to do ~ for someone below you (a child, junior, pet, plant) — the down-directed benefactive, blunter than てあげる

② ②: I'll damn well do it — a defiant/determined 'just you watch' nuance, the speaker asserting their own will

① ①

弟おとうとに宿題しゅくだいを教おしえてやった。

I taught my little brother his homework.

② ②

今度こんどこそ絶対ぜったいに勝かってやる。

This time I'll win for sure — just watch me.

① ①

毎朝まいあさ犬いぬを散歩さんぽに連つれて行いってやる。

Every morning I take the dog for a walk.

子こどもに絵本えほんを読よんでやった。

I read my child a picture book.

② ②

見みていろ、必かならず成功せいこうしてやる。

Just watch — I'll make it work, no matter what.

Sense ① is condescending if the recipient is an equal or a superior — never say 先生せんせいを手伝てつだってやる. Use てあげる for equals, お〜する for superiors. The whole form is rough/casual.

Sense ② grows out of sense ①'s assertiveness — the speaker 'does it (for the world / in spite of you)', a common line in sports and shōnen manga.

Jengo shows てやる 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.