dallyeoge yaksok naljjareul jeogeo dueosseoyo.

에 marks the place where the action happens or where something gets put.

So 달력에 means on the calendar or in the calendar, depending on how natural it sounds in English.

In this sentence:

• 달력에 = on the calendar
• 날짜를 적다 = write down the date

So the structure is basically:

• [place] + 에
• [thing written] + 를
• [verb]

약속 날짜 is a noun + noun combination.

• 약속 = appointment, promise, plan
• 날짜 = date

Together, 약속 날짜 means the appointment date or the date of the appointment.

There is no particle after 약속 because it is directly modifying 날짜, like a compound noun in English:

• appointment date
• meeting time
• school bus

Korean often does this without using 의.

So:

• 약속 날짜 = natural
• 약속의 날짜 = possible in theory, but unnatural here

Because 날짜 is the direct object of 적다.

The thing being written is the object, so it takes 을/를.

Here:

• 달력에 = where it was written
• 약속 날짜를 = what was written
• 적어 두었어요 = wrote it down and left it there / wrote it down in advance

A helpful way to think about it:

• 에 answers where
• 를 answers what

This is one of the most important points in the sentence.

• 적었어요 = wrote it
• 적어 두었어요 = wrote it down and left it there for later / wrote it in advance

The grammar -아/어 두다 adds the idea of:

• doing something beforehand
• doing something for future convenience
• leaving the result in place

So 달력에 약속 날짜를 적어 두었어요 does not just mean I wrote the appointment date on the calendar. It has the extra nuance of:

• I wrote it down so I wouldn’t forget
• I put it on the calendar and left it there as a reminder
• I already took care of that

It comes from:

• 적다 = to write down
• 적어 두다 = to write down and leave it that way / write down in advance
• 적어 두었어요 = polite past form

Step by step:

1. 적다
2. 적어
3. 두다
4. 두었어요 = past polite of 두다
5. Combine them: 적어 두었어요

In everyday speech, this is often contracted:

• 적어 두었어요 → 적어 뒀어요

So you may hear:

• 달력에 약속 날짜를 적어 뒀어요

That means the same thing.

Very often, yes, but not only that.

-아/어 두다 usually gives one or both of these ideas:

1. Do something in advance

   • for later use
   • for preparation
   • so you do not forget

2. Leave something in a certain state

   • after doing the action, the result remains

In this sentence, both ideas fit well:

• the speaker wrote the date down in advance
• and it remains written on the calendar

That is why this form feels very natural here.

Korean often leaves out the subject when it is already understood from context.

So this sentence does not explicitly say:

• I
• we
• she
• my mom

But in most normal situations, listeners can tell from context.

In many cases, learners mentally supply:

• I wrote the appointment date on the calendar

That is probably the most natural default interpretation, but grammatically the subject is simply omitted.

This is the standard polite style, often called -어요 style.

It is:

• polite
• natural in daily conversation
• very commonly used

It is less formal than 적어 두었습니다, but more polite than 적어 뒀어.

So the style level is appropriate for:

• everyday conversation
• polite speech
• most neutral situations

Yes, you can say 약속 날짜를 달력에 적어 두었어요 too.

Korean word order is more flexible than English, especially with particles marking each role.

Both of these are natural:

• 달력에 약속 날짜를 적어 두었어요
• 약속 날짜를 달력에 적어 두었어요

The difference is mostly about emphasis or information flow, not basic meaning.

The original sentence starts with 달력에, which makes the location feel slightly more prominent first:

• As for the calendar / on the calendar, I wrote down the appointment date.

Yes, 적어 두었어요 is the standard spaced form.

That is because 두다 is functioning as an auxiliary verb here, and Korean normally writes this separately:

• 적어 두었어요

In real life, you may also see or hear the contracted form:

• 적어 뒀어요

So:

• 적어 두었어요 = standard full form
• 적어 뒀어요 = common spoken/written contraction

Yes, in many situations you could.

• 적다 often has the nuance of write down, note down
• 쓰다 is the broader verb to write

So:

• 달력에 약속 날짜를 적어 두었어요
• 달력에 약속 날짜를 써 두었어요

Both can work.

However, 적다 often sounds especially natural when you mean:

• jotting something down
• recording information
• making a note

That is why 적어 두었어요 fits very well in this sentence.