Grip belirtileri dün başladı.

This is a common Turkish noun phrase pattern.

• grip = flu
• belirti = symptom
• grip belirtileri = flu symptoms

In Turkish, when one noun describes another noun, you often get a noun compound:

• first noun: stays plain → grip
• second noun: usually gets a possessive ending → belirtileri

So grip belirtileri literally works like flu its-symptoms, but in natural English we just say flu symptoms.

Because in this compound, the second noun takes a special ending.

Base word:

• belirti = symptom

Plural:

• belirtiler = symptoms

In the compound grip belirtileri, the second word also gets the 3rd person possessive ending:

• belirti + ler + i → belirtileri

So here belirtileri means something like its symptoms / the symptoms of flu.

This is very normal in Turkish noun compounds:

• ev kapısı = house door / the door of the house
• araba anahtarı = car key
• grip belirtileri = flu symptoms

Both are possible, but they are used a little differently.

• grip belirtileri = flu symptoms, the general compound form
• gribin belirtileri = the symptoms of the flu, more explicitly possessive

So:

• Grip belirtileri dün başladı. = Flu symptoms started yesterday.
• Gribin belirtileri dün başladı. can sound more like The symptoms of the flu started yesterday.

In everyday Turkish, the shorter compound form grip belirtileri is very common.

Also note the sound change:

• dictionary form: grip
• before a vowel in some forms: gribin

This is because final p often becomes b before a vowel.

Başladı is the simple past tense.

It comes from:

• başlamak = to start, to begin

Breakdown:

• başla- = verb stem
• -dı / -di / -du / -dü = past tense ending

So:

• başladı = it started / started

Because the subject is already in the sentence, Turkish does not need a separate word for it or they here.

The dictionary form is başlamak = to start / to begin.

In this sentence, it is used intransitively, meaning the symptoms themselves started.

• Grip belirtileri dün başladı. = Flu symptoms started yesterday.

If you want to start something, Turkish often uses başlatmak:

• Doktor tedaviyi başlattı. = The doctor started the treatment.

So:

• başlamak = to start / begin
• başlatmak = to start something, to cause to begin

This is very normal in Turkish.

With plural subjects, especially non-human ones, Turkish often uses a singular verb.

So:

• Grip belirtileri dün başladı. = natural
• Grip belirtileri dün başladılar. = not natural here

Turkish does not match plural subjects and verbs in exactly the same way English does.

A useful rule:

• with people, plural verbs are more possible
• with things/abstract nouns/non-human nouns, singular verbs are very common and usually preferred

Since symptoms are not human, başladı is the expected form.

Dün means yesterday.

It is a time adverb, and it tells you when the action happened.

So:

• Grip belirtileri dün başladı. = Flu symptoms started yesterday.

It does not need any extra ending here.

Yes. Turkish word order is flexible, although some orders sound more neutral than others.

The sentence you have:

• Grip belirtileri dün başladı.

Other possible orders:

• Dün grip belirtileri başladı.
• Grip belirtileri başladı dün. (possible, but less neutral in many contexts)

The most neutral position for the verb in Turkish is often at the end, so both of these are very natural:

• Grip belirtileri dün başladı.
• Dün grip belirtileri başladı.

Changing the order can slightly change the emphasis:

• Dün first puts more focus on yesterday

Turkish does not have a direct equivalent of English the.

So Turkish often leaves definiteness to:

• context
• word order
• possessive structures
• case marking

That means Grip belirtileri dün başladı could be understood in English as:

• Flu symptoms started yesterday
• The flu symptoms started yesterday

The exact English translation depends on context, but Turkish often does not need a separate article.

A simple pronunciation guide:

• Grip → roughly greep
• belirtileri → beh-leer-tee-leh-ree
• dün → a front rounded vowel, like German ü or French u
• başladı → bash-luh-duh

A rough full pronunciation:

• greep beh-leer-tee-leh-ree duun bash-luh-duh

A few notes:

• ş sounds like sh
• ı is not English i; it is a dull vowel, somewhat like the second vowel in sofa, but farther back
• ü does not exist in standard English, so learners usually need practice with it

Yes, you may hear that too.

• belirti = symptom, sign
• semptom = symptom

So:

• Grip belirtileri = flu symptoms
• Grip semptomları = flu symptoms

Both are understandable, but belirti is often the more everyday Turkish word, while semptom can sound a bit more medical or technical.

Not exactly. In Turkish, grip is still a noun meaning flu.

But Turkish often uses one noun to modify another noun in a compound:

• grip belirtileri = flu symptoms
• okul kapısı = school door
• yaz tatili = summer holiday

So even though it feels adjective-like in English, grammatically it is better to think of grip here as the first noun in a noun compound.