Learn Hardcore Japanese: keeki no nokori wo reizouko ni irete oku. - I will put the rest of the cake in the refrigerator.

〜ておく means “do something now so that its result remains for later” or “leave something as (you put) it.” Here it means “put it in the fridge and keep it there (for later convenience),” not just “put it in.” It adds preparation/lasting-result nuance.

Why is に used after 冷蔵庫? Can I use で or へ?

に marks the target/destination or container you move something into, which fits 入れる.
で marks the place where an action happens; with 入れる, that would be odd, because the important relation is destination, not the action’s venue.
へ is a direction marker and is rarely used with 入れる; stick with に for natural speech.

Why is を used after 残り?

Because 入れる is a transitive verb that takes a direct object. ケーキの残り is what gets put, so it takes を.

Who is the subject here, and why is it missing?

Japanese often omits the subject when it’s obvious from context. In everyday use, this likely means “I/we’ll (put it in and keep it),” but it could be “you/he/she/they” depending on the situation.

Is there a difference between ケーキの残り and 残りのケーキ?

Both are natural and usually interchangeable.

ケーキの残り = “the remainder of the cake” (emphasizes the leftover portion as a quantity).
残りのケーキ = “the leftover cake” (emphasizes the cake that remains as an item/set).

Can I say 残ったケーキ instead? Any nuance difference?

Yes. 残ったケーキ (cake that remained) is also natural and slightly highlights the result of having some cake left after an event. In most contexts it’s fine to use this in place of ケーキの残り.

Why is おく in hiragana? Is it the same as 置く?

It’s the same verb historically, but when used as the auxiliary 〜ておく, it’s typically written in hiragana to signal its grammaticalized meaning (“do in advance/leave as is”). Writing 入れて置く isn’t wrong, but 入れておく is the standard style.

How is 入れておく different from 入れてある?

入れておく: from the doer’s perspective, “put it in and keep it there (for later benefit).”
入れてある: from the item/state perspective, “it is (already) in the fridge because someone put it there (on purpose).” Example nuance: もう冷蔵庫に入れてある = “It’s already in the fridge (set there).”

How can I make this polite, past, negative, or a request?

Polite: 入れておきます
Past (casual): 入れておいた
Polite past: 入れておきました
Negative (casual): 入れておかない
Request: 入れておいてください
Volitional/self-intent: 入れておこう (“I’ll go ahead and put it in”)

Can I shorten 入れておく to 入れとく?

Yes. In casual speech 〜ておく often contracts to 〜とく: 入れとく. Other examples: しておく → しとく, 洗っておく → 洗っとく.

Why not use 入る instead of 入れる?

入る is intransitive (“to go/fit/enter”), while 入れる is transitive (“to put in”). ケーキの残りが冷蔵庫に入る means “the leftover cake fits/goes in (by itself),” not “put it in.”

Is 冷蔵庫の中に better than 冷蔵庫に?

Both are correct. 冷蔵庫の中に explicitly says “inside the fridge,” while 冷蔵庫に already implies “into the fridge (inside).” Use の中に when you want to emphasize “inside.”

Are spaces like in the example normal in Japanese?

No. Standard Japanese doesn’t use spaces between words. A typical native-style writing would be: ケーキの残りを冷蔵庫に入れておく。 Textbooks add spaces to aid learners.

How do I read and pronounce the parts?

ケーキ: keeki (long “ee”)
残り: nokori
を: pronounced “o”
冷蔵庫: reizōko (rei-zō-ko; long “ō”)
に: ni
入れておく: irete oku (often flows as iret’oku in casual speech)

Could I use しまう here, like 入れてしまう?

〜てしまう marks completion (and sometimes regret/undesirability). 入れてしまう = “to (completely) put it in” or “end up putting it in,” which lacks the “keep for later” nuance. You may also hear the standalone verb 冷蔵庫にしまう (“put it away in the fridge”). For “put and keep (for later),” 入れておく is the best match.

Can I use は instead of を (e.g., ケーキの残りは冷蔵庫に入れておく)?

Yes. は topicalizes: “As for the leftover cake, (I’ll) put it in the fridge (and keep it there).” It shifts focus to the topic. Don’t use を and は together on the same noun phrase; は replaces を in such topicalization.

Can I drop the object or the location if they’re obvious?

Yes. Both are commonly omitted when clear by context:

冷蔵庫に入れておくね。 (“I’ll put it in the fridge.”)
これ、入れておくね。 (“I’ll put this away [in its proper place].”) Japanese relies heavily on context-driven omission.