daizina yotei ha tetyou ni kaite, sumaho ni mo irete okimasu.

Because 大事 is a na-adjective in Japanese, not an i-adjective.

• When a na-adjective comes before a noun, you add な:
  • 大事な予定 = an important plan/schedule
• Compare:
  • 静か → 静かな部屋
  • 便利 → 便利なアプリ

So 大事な is the correct form before 予定.

予定 means something like a plan, schedule, or appointment. It is a broad word for something you intend to do or that is scheduled.

• 予定 = a planned event, schedule item, arrangement
• 約束 = a promise, or an appointment made with someone

So in this sentence, 予定 fits well because the speaker is talking about important things they want to record in a planner and phone. It does not have to mean a promise with another person.

は marks 予定 as the topic of the sentence.

So 大事な予定は means something like:

• As for important plans...
• When it comes to important appointments...

The sentence then says what the speaker does with those important plans.

This is very natural in Japanese, where the topic is often introduced first and then described.

Here, に marks the place where the information is entered or written.

• 手帳に書く = write it in a planner/notebook
• ノートに書く = write it in a notebook
• 紙に書く = write it on paper

In English, we often say write in or write on, but Japanese commonly uses に for the destination/location where the writing goes.

The も means also / too.

So:

• 手帳に書いて = write it in my planner
• スマホにも入れて = also put it in my smartphone

The particle combination にも is:

• に = into/in/on the smartphone
• も = also

So the speaker is saying they record it in both places.

入れる literally means to put in, but it is often used for entering information into a device, app, or system.

Here, スマホに入れる means something like:

• enter it into my phone
• put it into my phone calendar
• save it on my phone

Japanese often uses 入れる in this broader sense for data, appointments, reminders, and so on.

The て-form is being used to connect actions in sequence.

So:

• 書いて、入れておきます means
• I write it down, and also enter it...

It links the first action to the second one smoothly.

A rough breakdown:

• 書く → 書いて
• 入れる → 入れて

Then the sentence continues after each one.

〜ておく / 〜ておきます often means:

• do something in advance
• do something to prepare
• do something just in case

So 入れておきます means the speaker enters it into their smartphone ahead of time so it will be there when needed.

In this sentence, the nuance is:

• I make sure to put important plans into my phone beforehand.

This is one of the most useful grammar patterns in everyday Japanese.

Grammatically, 〜ておく is attached only to 入れて in this sentence, so strictly speaking only the second verb has that form.

However, in natural interpretation, the whole sentence suggests a habit of making sure important plans are recorded. So even though 書く does not explicitly have 〜ておく, the overall feeling is still one of preparation and careful record-keeping.

If someone wanted to make both verbs explicitly parallel, they might say something like:

• 手帳に書いて、スマホにも入れておきます
• or in a more expanded form, 手帳に書いておいて、スマホにも入れておきます

But the original sentence is completely natural.

The non-past polite form here often describes a habit or regular practice.

So the sentence means something like:

• I write important plans in my planner and also put them in my phone.
• That’s what I do with important plans.

It is not necessarily talking about one single future action. It can describe the speaker’s usual routine.

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

Here, the natural understood subject is I:

• (私は) 大事な予定は手帳に書いて、スマホにも入れておきます。

In English, you usually need the subject, but in Japanese it is very common to omit it if the listener can infer it.

Yes. 手帳 usually means a planner, datebook, or schedule book rather than just any notebook.

So 手帳に書く strongly suggests writing appointments or plans in a personal planner.

By contrast:

• ノート = notebook
• 手帳 = planner/datebook/schedule book

That makes 手帳 especially appropriate with 予定.