て形の表現N4
〜ておく（〜とく）
to do... in advance / to leave... as is
Structure
V-て形 + おく（口語：〜とく）
Explanation
Attached to the te-form of verbs, 〜ておく expresses: ① doing something in advance as preparation; ② leaving something in a certain state. In casual speech it contracts to 〜とく (e.g. やっておく→やっとく).
Examples
旅行の前に、ホテルを予約しておきます。
I'll reserve a hotel before the trip.
友達が来るので、部屋をきれいにしておきました。
Since a friend is coming, I cleaned the room in advance.
Note
〜ておく often appears with preparation-related words. 〜とく is casual. It can also mean "to leave as is" (e.g., そのままにしておく).