kono tetyou ni asita no yotei wo kaite okimasu.

In this sentence, に marks the place where the writing goes.

• この手帳に書く = write in this planner / write into this planner

For actions like writing, に often marks the target location where something is added:

• ノートに名前を書く = write your name in a notebook
• 紙に住所を書く = write an address on paper

A learner might expect で because it can mark a place of action, but here the important idea is not just where you are doing the action, but where the written content is being put. So に is natural.

明日の予定 is a noun phrase made of:

• 明日 = tomorrow
• の = links one noun to another
• 予定 = plans, schedule, arrangements

So literally it is:

• tomorrow's plans
• the schedule for tomorrow

The particle の is doing the same kind of job as 's or for in English, depending on the context.

Because 明日 and 予定 are both nouns here, and Japanese usually uses の to connect nouns.

So:

• 明日 = tomorrow
• 予定 = schedule / plans
• 明日の予定 = tomorrow's schedule / plans for tomorrow

Without の, the phrase would not sound correct in standard Japanese.

Japanese nouns usually do not change form for singular vs. plural.

So 予定 can mean:

• a plan
• plans
• a schedule
• arrangements

The exact number is understood from context. In English, you often have to choose between plan and plans, but in Japanese the same noun can cover both.

This is one of the most important parts of the sentence.

• 書きます = write
• 書いておきます = write in advance / write down and leave it there for later

The pattern ておく means doing something beforehand so it will be ready, useful, or taken care of later.

So:

• 明日の予定を書きます = I will write tomorrow's schedule/plans.
• 明日の予定を書いておきます = I will write down tomorrow's schedule/plans in advance.

This gives the sentence a feeling of preparation.

Yes, historically it comes from 置く meaning to put or to place, but in the grammar pattern 〜ておく, it works more like an auxiliary expression.

So 書いておく does not literally mean write and put. It means:

• do the writing in advance
• write it down and leave it ready

In casual speech, 〜ておく is often shortened:

• 書いておく → 書いとく
• 書いておきます → 書いときます

But in your sentence, the full polite form 書いておきます is being used.

を marks the direct object of the verb.

Here, the thing being written is:

• 明日の予定 = tomorrow's plans / schedule

So:

• 明日の予定を書く = write tomorrow's plans

That is why 予定を appears in the sentence.

Japanese often leaves out the subject when it is obvious from context.

So even though the sentence does not say I, the natural interpretation is often:

• I will write down tomorrow's schedule in this planner in advance.

Depending on context, it could also mean we or another understood person, but in isolation, I is the most natural English translation.

この means this, but it must be followed by a noun.

So:

• この手帳 = this planner / this notebook

You cannot usually use この by itself. If you want to say this one, you would use これ instead.

Compare:

• この手帳 = this planner
• これ = this

手帳 usually means a small personal notebook, planner, datebook, or agenda-type book.

In this sentence, it most naturally sounds like:

• a planner
• a schedule book
• a personal datebook

That fits well with 明日の予定 because people commonly write schedules and plans in a 手帳.

Not completely. Japanese word order is more flexible than English, as long as the particles make the roles clear.

The original sentence is:

• この手帳に 明日の予定を 書いておきます。

You could also hear things like:

• 明日の予定をこの手帳に書いておきます。

Both are natural. The particles に and を tell you what each part is doing, so the meaning stays clear.

However, the verb usually comes at the end of the sentence.

The non-past form in Japanese can express:

• future actions
• habitual actions
• present/future intentions

Here, 書いておきます means something like:

• I’ll write it down in advance
• I’m going to write it down beforehand

Even though tomorrow is mentioned, the writing itself may happen before tomorrow. The non-past polite form is still the normal way to say that.

Normally, 明日の予定 means the schedule/plans for tomorrow, not plans that will be written tomorrow.

That is because 明日 is directly connected to 予定 with の:

• 明日
  • の
    • 予定
• tomorrow's plans

So 明日 modifies 予定, not 書いておきます.

If you wanted to emphasize that the writing happens tomorrow, the sentence would need a different structure, such as putting 明日 closer to the verb.

It is in the polite form because it ends with ます:

• 書いておきます

A plain-form version would be:

• この手帳に明日の予定を書いておく。

The polite version is appropriate in normal conversation, with strangers, in class, or in many everyday situations.