isogasii node, tomodati no messeezi dake saki ni kidoku ni site, henzi ha ato de kakimasu.

ので gives a reason: because / since.

• 忙しいので = because I’m busy
• The sentence is explaining the reason for the actions that follow.

Compared with から, ので usually sounds a bit more neutral, softer, and more explanatory.

• 忙しいから、… = because I’m busy...
• 忙しいので、… = because I’m busy... (slightly more formal or gentle)

Both are possible, but ので fits well in calm explanations like this.

Because 忙しい is an い-adjective.

With い-adjectives, you attach ので directly:

• 忙しいので

You do not add だ or な.

Compare:

• 忙しいので = because it’s busy / because I’m busy
• 学生なので = because I’m a student
• 静かなので = because it’s quiet

So:

• い-adjective → 忙しいので
• noun / な-adjective → 〜なので

In natural English, it usually means a message from my friend.

Literally, A の B often works like B of A or A’s B, but in Japanese this relationship is broad. So:

• 友達のメッセージ can literally look like friend’s message
• but in real usage it usually means the message from a friend

The speaker is probably talking about a message they received from a friend, not a message their friend owns in some abstract sense.

だけ means only / just.

Here it follows 友達のメッセージ, so it applies to that noun phrase:

• 友達のメッセージだけ = only my friend’s message(s)

So the idea is:

• I’ll mark only my friend’s message as read first
• not other messages

It does not mean I’ll only do this first. The only specifically limits the friend’s message(s).

先に means first / beforehand / in advance.

後で means later.

They create a time contrast:

• 先に既読にして = mark it as read first
• 返事は後で書きます = as for the reply, I’ll write it later

So the speaker is separating two actions:

1. first, mark the message as read
2. later, write the reply

This is a very common Japanese way to organize actions in sequence.

既読 means already read / read status.

In messaging apps, 既読にする means to mark something as read or to make it show as read.

The pattern is:

• Noun + にする = make it into ~ / turn it into ~

So:

• 既読にする = make it “read”
• 既読にして = the て-form, linking it to the next action

This is not exactly the same as simply reading the message. It emphasizes the status becoming read, especially in an app or chat context.

Not exactly.

• 読む = to read
• 既読にする = to mark as read / make the status read

In real life, these may happen together, but the nuance is different.

For example:

• メッセージを読む = read the message
• メッセージを既読にする = mark the message as read

In app-related language, 既読 often refers to the read-status label, not just the act of understanding the message.

して is the て-form of する.

The て-form links actions together. In this sentence:

• 既読にして、返事は後で書きます
• mark it as read, and write the reply later

It often implies sequence:

1. do action A
2. then do action B

Here, the exact flow is:

• mark only the friend’s message as read first,
• then write the reply later

So して is connecting the first action to the second.

Using は here adds a contrastive or topic-like feeling.

• 返事を後で書きます = I’ll write the reply later
• 返事は後で書きます = as for the reply, I’ll write it later

This matches the contrast in the sentence:

• the message gets marked as read first
• the reply, on the other hand, gets written later

So は helps highlight 返事 as a separate item being contrasted with the earlier action.

Yes, 返事 means reply / response, but in Japanese it’s very natural to say:

• 返事を書く = write a reply
• 返事をする = reply / give a response

Both can be possible depending on context, but 書きます fits especially well when the reply is a written message, like in texting or chat.

So here:

• 返事は後で書きます = I’ll write the reply later

This sounds natural because the context is messaging.

Yes, the subject is omitted, which is very common in Japanese.

From context, the subject is understood as the speaker:

• (私は)忙しいので、…
• (私は)返事は後で書きます。

So the natural interpretation is:

• I’m busy, so I’ll mark only my friend’s message as read first, and I’ll write the reply later.

Japanese often leaves out I, you, he, etc. when they are obvious from context.

Good question. You might expect something like:

• 友達のメッセージだけを既読にして

That would be grammatically possible.

But in everyday Japanese, を is often omitted before expressions like 〜にする, especially when the meaning is already clear.

So:

• 友達のメッセージだけ先に既読にして
• 友達のメッセージだけを先に既読にして

Both are understandable, but the version without を sounds natural and lighter in casual written Japanese.

Here, 後で is the natural choice.

• 後で = later / afterwards
• 後に often sounds more formal, more rigid, or more like after ~ in set sequencing

In casual everyday speech:

• 返事は後で書きます = very natural

Using 後に here would sound less natural in ordinary conversation or texting.

So for I’ll do it later, 後で is the usual expression.

It could be understood either way depending on context.

• 友達のメッセージ can refer to a message from a friend
• or messages from a friend

Japanese nouns usually do not show singular/plural explicitly.

So the context decides:

• if there is one specific message, then message
• if there are several, then messages

The grammar itself does not force one or the other.

The commas help readability and show pauses:

• 忙しいので、
• …して、
• 返事は後で書きます。

They make the sentence easier to process, especially because it has a reason clause and a linked action.

They are helpful, but punctuation in Japanese is often more flexible than in English. The commas are not always strictly required, though in this sentence they make it much clearer.

Yes, it is natural, especially in a modern messaging/app context.

Important natural points are:

• 忙しいので for giving a reason
• 先に and 後で for time order
• 既読にする for app/message read-status language
• 返事は for contrast: read now, reply later

So it sounds like a realistic thing someone might say when they are busy and managing messages.