tomodati no messeezi ha mou kidoku ni sita kedo, henzi ha mada kaite inai.

既読 is read きどく. In messaging-app language, it means read / already read, especially the status that shows a message has been opened.

So in this sentence, it does not just mean I read the message in a general sense. It has the nuance of the message is now marked as read.

This is the pattern noun + にする, which means to make something into that state.

So:

• 既読 = read status
• にする = make it into / set it to

Therefore, 既読にした literally means made it read, or more naturally, marked it as read / opened it so it shows as read.

This is similar to patterns like:

• 静かにする = make it quiet / be quiet
• オンにする = turn it on

の links two nouns. Here it shows the relationship between 友達 and メッセージ.

So 友達のメッセージ means:

• my friend’s message
• a friend’s message
• the message from a friend

Which one sounds best in English depends on context, but grammatically it is just friend + message connected by の.

Here, は is marking them as topics, and it also creates a feeling of contrast.

The sentence is basically contrasting two things:

• As for the message, I already marked it as read
• As for the reply, I haven’t written it yet

Using は twice makes that contrast very clear. It is like saying:

• The message — done
• The reply — not done yet

If you used を, it would sound less contrastive and more like a plain object marker.

They form a very common contrast:

• もう with a positive verb = already
• まだ with a negative verb = not yet

So:

• もう既読にした = already marked it as read
• まだ書いていない = have not written it yet

This pairing is extremely common in Japanese.

けど means but, though, or although, depending on context.

In this sentence, it connects two clauses that contrast with each other:

• I already marked the message as read,
• but I haven’t written a reply yet.

It is a very common, natural way to join sentences in everyday Japanese. It often sounds a little softer and more conversational than a more direct English but.

書いていない means have not written / am not in the state of having written. In natural English here, that is haven’t written it yet.

This fits because the speaker is talking about the current situation: the reply still has not been written.

By contrast, 書かなかった means didn’t write in a simple past sense. That sounds more like talking about a completed past event, not a reply that is still pending now.

So まだ書いていない is the natural choice for not yet written.

In Japanese, 返事を書く is very natural when the reply is something written, such as a text, message, email, or letter.

So even if English might focus on send a reply, Japanese often focuses on write a reply.

Other expressions are possible too, such as:

• 返信していない = haven’t replied
• 返事していない = haven’t answered / haven’t replied

But 返事はまだ書いていない sounds very natural when the speaker means the reply text itself has not been written yet.

The subject is omitted because it is understood from context. This is very normal in Japanese.

The sentence naturally implies:

• I already marked my friend’s message as read,
• but I haven’t written a reply yet.

Japanese often leaves out subjects like I, you, or he/she when they are obvious.

It can be either, depending on context.

Japanese does not use articles like a and the, and words like my are often omitted when they are obvious.

So 友達のメッセージ could be understood as:

• my friend’s message
• a friend’s message

In many real situations, English would naturally translate it as my friend’s message, even though my is not stated explicitly in Japanese.