N4
ておく
To do something in advance
寝る前に食器を洗っておく。
Before sleeping, I will wash the dishes in advance.
Explanation
「ておく」 has 3 uses
1Doing in advance
Doing something now so it's ready later: 「会議の前に資料を読んでおく」 read the papers before the meeting.
ここにリモコンを置いておくね。
I'll leave the remote here.
寝る前に食器を洗っておく。
Before sleeping, I will wash the dishes in advance.
2Putting things right after
Tidying up after something is done: 「食べた後、お皿を洗っておく」 wash the dishes after eating.
使ったはさみは、引き出しに戻しておいてね。
Put the scissors back in the drawer when you're done.
飲み会のあと、コップを洗っておいた。
After the party, I washed the glasses.
3Leaving it as it is
Leaving something the way it is: 「窓を開けておいて」 leave the window open.
暑いから、窓を開けておいて。
It's hot — leave the window open.
まだ使うから、パソコンはつけておいてください。
I still need it, so leave the computer on.
Mixed up with
The short forms almost always appear in the listening section: 「買っとくね」, 「置いといて」. If you only know 「ておく」 you may miss them.