This structure uses the te form with the verb “to put” (置く) to express that you will perform an action and then leave it in that state (typically in preparation for something in the future). For example, you might have guests coming in a few hours so you say to your partner, “I’ll put the wine in the fridge” or you might be late for a meeting so you give someone a call to let them know in advance. It doesn’t always have a direct equivalent in English.
Note: When used as auxiliary grammar like this, おく is almost always written in hiragana.
準備しておく。
Prepare in advance.
遅れそうだからジェームズに電話しておくね。
It looks like we’re going to be late so I’ll give James a call in advance.
ビールを冷やしておく。
Chill the beers (for later).
お客さんが３時ぐらいに来るので、会議室を予約しておきましょう。
The clients are coming around 3 o’clock so let’s book a meeting room in advance.
予備の電池を用意しておく。
Keep a spare battery ready (just in case).
Leaving Things As They Are
Another major function of this grammar is to express leaving something in its current state, or letting someone be. This is heavily tested on the JLPT.
窓を開けておいてください。
Please leave the window open.
そのままにしておいて！
Leave it exactly as it is! (Don’t touch it!)
Casual Abbreviations (〜とく / 〜どく)
In casual conversation, the ～ておく almost always gets abbreviated to 〜とく.
準備しとく。
I’ll prepare in advance.
買っといてよかったよね。
It’s a good thing I bought it in advance [because the price has now gone up].
Critical JLPT Listening Trap: If the verb’s te-form ends with a voiced で (such as 飲む → 飲んで, or 読む → 読んで), the abbreviation also becomes voiced: 〜どく.