Virüs belirtileri sonunda yatıştı.

A natural breakdown is:

• virüs = virus
• belirtileri = symptoms / virus symptoms
• sonunda = finally, in the end, eventually
• yatıştı = subsided, calmed down, settled

So the sentence structure is basically:

• subject: Virüs belirtileri
• adverb: sonunda
• verb: yatıştı

Turkish normally puts the verb at the end, so this word order is very natural.

Because virüs belirtileri is a noun compound.

• belirti = symptom
• belirtiler = symptoms
• virüs belirtisi = virus symptom
• virüs belirtileri = virus symptoms

In Turkish, when one noun modifies another noun like this, the second noun usually takes a special ending, often called the compound or possessive ending. That is why you get belirti → belirtisi, and in the plural belirtileri.

So virüs belirtileri does not literally work like English adjective + noun. It is more like a Turkish noun+noun structure meaning virus symptoms or the symptoms of the virus, depending on context.

Not here.

That is a very common confusion, because -leri can look like an accusative form in other sentences. But in this sentence, belirtileri is the subject, not the object.

A key clue is the verb yatışmak. It is an intransitive verb, so it does not take a direct object here. The symptoms are the thing doing the action of subsiding.

So in this sentence, belirtileri is part of the noun phrase virüs belirtileri, not an accusative-marked object.

Both are possible, but they are not exactly the same in feel.

• virüs belirtileri = virus symptoms / viral symptoms
  This is an indefinite noun compound. It sounds more like a category or type.
• virüsün belirtileri = the symptoms of the virus
  This is a genitive-possessive structure. It sounds more explicitly like a specific virus whose symptoms you are talking about.

In real usage, the difference can be subtle, and context matters a lot. In many situations, virüs belirtileri is perfectly natural and idiomatic, especially when talking about symptoms associated with a virus.

Because in Turkish, non-human plural subjects often take a singular verb.

So:

• Virüs belirtileri sonunda yatıştı = natural
• Virüs belirtileri sonunda yatıştılar = usually not natural here

This is different from English. English requires symptoms subsided, but Turkish commonly uses third-person singular with plural non-human subjects.

If the subject were people, plural agreement would be more likely.

Here sonunda means finally, eventually, or in the end.

It comes from son meaning end, so the literal sense is something like at the end. But in everyday Turkish it is very often used adverbially to mean that something happened after some delay or after a period of waiting.

So in this sentence, sonunda suggests that the symptoms lasted for a while and then finally subsided.

Yes, it is related historically, but as a learner it is best to treat yatışmak as its own verb.

• yatmak = to lie down
• yatışmak = to calm down, settle down, subside
• yatıştı = calmed down / subsided

With things like:

• pain
• symptoms
• inflammation
• panic
• anger

yatışmak often means to subside or to ease off.

So Virüs belirtileri sonunda yatıştı means the symptoms settled down or subsided. It does not mean they literally lay down.

-tı is the simple past tense ending.

So:

• yatışır = it subsides / it tends to subside
• yatıştı = it subsided

In this sentence, it shows that the subsiding happened in the past and is viewed as a completed event.

Yes. Turkish word order is flexible, although the verb usually stays at the end.

For example:

• Virüs belirtileri sonunda yatıştı. = neutral, natural
• Sonunda virüs belirtileri yatıştı. = puts more emphasis on finally
• Virüs belirtileri yatıştı sonunda. = possible in speech, but less neutral

So the original sentence is a normal, standard way to say it.