sakuya okutta messeezi ha mada kidoku ga tuite imasen.

Because 送ったメッセージ is a relative clause in Japanese.

It works like:

• 昨夜送ったメッセージ
• the message [I] sent last night

In Japanese, the describing clause comes before the noun it modifies. So instead of saying something like the message that I sent, Japanese says [sent] message.

Here:

• 昨夜 = last night
• 送った = sent
• メッセージ = message

So 昨夜送ったメッセージ literally has the structure last-night sent message.

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

So 送った by itself does not explicitly say who sent it. In this sentence, the natural interpretation is the message I sent last night, because that is what makes sense in context.

This kind of omission is extremely common in Japanese. English usually needs a subject, but Japanese often does not.

Because メッセージ is already the thing being sent.

Compare:

• Normal clause: 昨夜 メッセージを 送った = I sent a message last night
• Relative clause: 昨夜 送った メッセージ = the message I sent last night

In a relative clause, the noun being modified sits outside the clause, so you do not keep the object particle inside as well.

So 送ったメッセージ is not missing を by accident. That is just how Japanese relative clauses work.

Many time expressions can be used without a particle.

So:

• 昨夜送った
• 昨日送った
• 今朝送った

are all natural.

With specific clock times, に is more common:

• 3時に送った

But general time words like 昨夜, 昨日, 今朝, 先週 often appear with no particle.

は marks 昨夜送ったメッセージ as the topic of the sentence.

So the sentence is basically saying:

• As for the message I sent last night, ...

Then the rest comments on that topic:

• まだ既読がついていません = it still does not have the read mark

Using は helps frame the sentence around that particular message.

With a negative form, まだ usually means still or not yet, depending on how you phrase it in English.

So:

• まだ 既読がついていません

means:

• it still has not been marked as read
• the read receipt has not yet appeared

This is a very common pattern:

• まだ来ていません = has not come yet / still hasn’t come
• まだ食べていません = haven’t eaten yet

So まだ + negative is an important pattern to remember.

既読 means already read, but in messaging apps it usually refers to the read status or read receipt.

It is read きどく.

So in this sentence, 既読 is not just the abstract idea of reading. It means the visible app status that shows the message has been read.

That is why a natural English translation is often something like:

• the message still has no read receipt
• the message still hasn’t been marked as read

Because the expression here is 既読がつく.

Literally, つく means things like:

• to attach
• to stick
• to be added
• to appear

So 既読がつく means something like:

• the read mark gets attached
• the read status appears

In that expression, 既読 is what appears or gets attached, so it takes が.

This is more natural than trying to think of 既読 as a direct object here.

That is because つく has several meanings, and here it means to become attached / to appear / to be added.

In app-related Japanese, people often say things like:

• 既読がつく = the read mark appears
• マークがつく = a mark gets added
• コメントがつく = a comment gets added

So 既読がつく is a very natural digital-context expression.

Because ている here describes the resulting state, not an action in progress.

Compare the nuance:

• 既読がつく = a read mark appears / gets attached
• 既読がついている = the read mark is on there now

So:

• 既読がついていません = the read mark is not on there / has not appeared

This sounds more natural for describing the current status of the message.

If you used つきません, it would sound more like a general statement such as it does not attach or it will not appear, which is not the main point here.

Not necessarily.

What it says directly is that the read receipt has not appeared.

In many apps, that strongly suggests the person has not opened or read it. But strictly speaking, the sentence is about the status marker, not the person’s actual mental act of reading.

So the nuance is:

• No read receipt yet
• therefore probably not read, but the sentence itself focuses on the app status

Yes. A few natural alternatives are:

• 昨夜送ったメッセージは、まだ既読になっていません。
• 昨夜送ったメッセージは、まだ読まれていません。

But the nuance changes a little:

• 既読がついていません focuses on the read mark appearing
• 既読になっていません focuses on the message not having entered the read state
• 読まれていません focuses more directly on not being read

In the context of messaging apps, 既読がついていません sounds very natural because people often talk specifically about whether the read indicator has shown up.

昨夜 can be read as さくや or ゆうべ.

Both mean last night.

In everyday conversation, ゆうべ is very common. In writing, さくや may feel a little more formal or written, but both are normal.

So this sentence could be read naturally either way depending on the speaker and situation.