~ておく / とく for preparation (Conjugation and nuance)?

Question

A few quick questions regarding ～ておく and the casual form ～とく Firstly, when changing from ～ておく to the more casual ～とく　I'm assuming the verb is first conjugated to the ～て　form then the ～て is dropped and replaced with とく and it can then be conjugated following the godan conjugation pattern as in the examples below. I.e. > 勉強して　→　勉強しておく　→　勉強しとく ～とく conjugates as godan such that 勉強しときます and　勉強しときました. In the case of verbs that have a ～で ending do they conjugate as どく？ I.e. > 読んで　→　読んでおく　→　読んどく ～どく　also conjugates as godan such that 読んどきます and 読んどきました Is my understanding correct here? Also, are there any nuances that make it outright different from ～つもりです in terms of preparing for something?　Is it simply that one of the sentences below feels more natural than the other? For example > 明日試験があるので今晩勉強しておきます vs 明日試験があるので今晩勉強するつもりです Thanks

Answer 1

Yes. The contraction of TE+OKU and DE+OKU is TOKU and DOKU. They would conjugate just like "OKU" (to place/put) - OKIMASU, OKANAI, etc. in all cases when you would need to conjugate it.

TSUMORI only implies intention, but TE-OKU implies that an action is taking place to PREPARE for a future action. Your examples don't really seem to have that much of a difference:

A: 明日試験があるので今晩勉強しておきます - Because there is a test tomorrow, I will study for it (to prepare for it.).

B: 明日試験があるので今晩勉強するつもりです - Because there is a test tomorrow, I intend to study for it.

Let's look at the past:

昨日の試験合格できました！ちゃんと勉強しておきましたから。- I passed yesterday's test because I studied for it properly.

昨日の試験失敗しました。勉強するつもりだけど、友達と遅くまで出かけるようになりましたから… I failed yesterday's test. I intended to study but I wound up staying out late so....

TSUMORI only implies intention and it isn't as "serious." Since TE+OKU has a sense of doing something for the future and maintaining that for the future, the utterance seems like it takes more foresight and planning.