ておく details

write + (and…) + do in advance

kaite oku

do in advance; do and leave it that way

meaning: write in advance

飲のみ物ものは俺おれが買かっとくよ。

nomimono wa ore ga kattoku yo.

I'll get the drinks beforehand. casual

旅行りょこうの前まえにホテルを予約しよやくしておく。

ryokou no mae ni hoteru o yoyakushite oku.

I book a hotel in advance of the trip. neutral

Examples

do in advance — set something up now so a later situation goes smoothly

飲のみ物ものは俺おれが買かっとくよ。

nomimono wa ore ga kattoku yo.

I'll get the drinks beforehand. casual

旅行りょこうの前まえにホテルを予約よやくしておいた。

ryokou no mae ni hoteru o yoyakushiteoita.

I booked a hotel in advance of the trip. neutral

会議かいぎまでに資料しりょうに目めを通とおしておいてください。

kaigi made ni shiryou ni me o tooshite oite kudasai.

Please look over the materials before the meeting. formal

とく — ておく→とく, でおく→どく in casual speech — 買っとく, 読んどく. Conjugates as a godan verb (買っといた, 覚えとけ).

A purpose is built in, so the subject must be able to intend — no weather, no accidents.

てみる
Both aim the action beyond itself — てみる does it to find out the outcome, ておく does it to serve a coming situation; both demand a willing agent, so 雨が降ってみる and 降っておく are equally impossible.

てしまう
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.
ていく
Casual speech collapses them to a one-mora difference — 買っとく "buy in advance" vs 買ってく "buy and go"; only context tells a listener which auxiliary just went by.
てある
Same preparation scene, opposite viewpoint — 冷やしておく names the preparatory act, agent on stage and any tense available; 冷やしてある names the state that act left behind, agent already offstage.