Right. -Te oku implies that you(or subject) do something intensionally, on purpose, in advance for coming situation. The formula is: te-form of verb + oku. If you want to conjugate for past tense etc., change "oku", not the main verb. See the examples: あさ　うちを　でる　まえに　ばんごはんの　よういを　しておきます。 I prepare my dinner before leaving home in the morning (so I can eat as soon as I come home). vs. あさ　うちを　でる　まえに　ばんごはんの　よういを　します。おふろにも　はいります。そうじも　します。 I prepare my dinner before leaving home in the morning. I also take bath. Also I clean the room. やまださん、このほんを　よんでおいてください。 Yamada-san, please read this book (because we gonna use it later/ because I'll ask about the book later/because it will help you in future, etc). vs. やまださん、このほんを　よんでください。 Yamada-san, please read this book (because it was interesting/ because I wrote it/ because it's homework, etc.). Depending on the context, -te oku may mean to do something and leave it there temporarily (still expecting the coming situation). Example: ちょっと　あついですね。まどを　しばらく　あけておきましょう。 It's a bit hot here. Let's keep the window open for a while (and close it later). vs ちょっと　あついですね。まどを　あけましょう。 It's a bit hot here. Let's open the window. Hope it helps!

This is not answer. Would you give me sample phases? Maybe, I can give you answer.