I was watching a Japanese TV drama and came across this sentence: そのへんのこと よ〜く念頭に置いといてね. I think といてね comes from ておいてね. Can anyone please tell me how do we use this ておいてね and when it is conjugates with 置く how it changes it?
You think right, that といて is a shortened, colloquial version of ておいて. In other words, ~とく is a casual version of ~ておく where basically て+お is merged into と.
More precisely the shortenings work as follows:
- ~ておいて / でおいて --> ~といて / どいて
- ~ておく / でおく --> ~とく / どく
Note also the negative case:
- ~ておかないで / でおかないで --> ~とかないで / どかないで
In your example, 置く has nothing special and just follows the above rules. Hence in a casual setting 置いておいてね becomes 置いといてね.
Briefly, the construction Verb (て-form) + おく is used when you want to prepare something for future (use) / do something beforehand / go ahead and do something.
- チケット、買っといてね = Buy the ticket (in advance, it will be used later).
Another use is to "do something and leave it for a while / let someone do something for a while". For example:
- 彼らには言いたいことを言わせておけばいい = Let them say whatever they want.
Finally, in your case, the meaning is to "put and keep something well in your mind" (and keep it there for future use. In other words, don't forget!).