Oyuncu kadrosu bugün açıklandı.

Because oyuncu kadrosu is a very common Turkish noun compound.

• oyuncu = actor / player
• kadro = staff, squad, lineup, cast
• oyuncu kadrosu = the cast / cast lineup

In Turkish, when one noun modifies another noun like this, the second noun often takes a 3rd person possessive ending. Here that ending is -su:

• kadro → kadrosu

So this does not necessarily mean his/her cast. In this structure, it is just part of how Turkish builds compounds.

A few similar examples:

• okul kapısı = the school door
• film müziği = film music / the music of the film
• oyuncu kadrosu = cast

So kadrosu here is best understood as part of the fixed expression oyuncu kadrosu.

Not necessarily. In Turkish, the first noun in a noun compound is often left in the singular, even if the meaning is plural in English.

So:

• oyuncu kadrosu literally looks like actor cast
• but naturally means cast or cast lineup

This is normal Turkish usage. English often uses singular nouns this way too:

• car door
• toothbrush
• student card

Turkish does the same kind of thing with noun compounds.

If you said oyuncular kadrosu, it would sound less natural in this context.

It is the 3rd person singular possessive ending, but in this sentence it is functioning as part of the compound noun pattern.

The form depends on vowel harmony and buffer consonants:

• base noun: kadro
• possessive/compound ending: -(s)I
• result: kadrosu

Why -su?

• the last vowel of kadro is o
• according to 4-way vowel harmony, that gives -u
• because the noun ends in a vowel, Turkish inserts s before the ending

So:

• kadro + (s)u → kadrosu

This is the same pattern you see in:

• araba kapısı = car door
• sınıf listesi = class list
• oyuncu kadrosu = cast

Because açıklandı means was announced, while açıkladı means announced.

Compare:

• açıkladı = he/she/it announced
• açıklandı = it was announced

Your sentence uses the passive form, because the focus is on the cast being announced, not on who announced it.

So:

• Oyuncu kadrosu bugün açıklandı. = The cast was announced today.

If you wanted to say The company announced the cast today, you could say something like:

• Şirket oyuncu kadrosunu bugün açıkladı.

There the verb is active: açıkladı.

It comes from the verb açıklamak = to explain, announce, disclose.

Here is the basic breakdown:

• açıkla- = verb stem
• -n- / -l- passive-related formation appears here as part of açıklanmak
• -dı / -di / -du / -dü = past tense

So:

• açıklanmak = to be announced
• açıklandı = was announced

For a learner, the most useful thing is to recognize:

• açıkladı = announced
• açıklandı = was announced

It is the simple past tense.

The ending -dı here marks past time:

• açıklanıyor = is being announced
• açıklanacak = will be announced
• açıklandı = was announced

So the sentence places the announcement in the past, and bugün tells you that this past action happened today.

In English, today can go with a past tense too:

• It was announced today.

Because Turkish does not have a direct equivalent of the English definite article the.

So Turkish often leaves definiteness to be understood from:

• context
• word order
• whether the noun is a known/specific thing
• the overall situation

In Oyuncu kadrosu bugün açıklandı, the noun is understood as a specific cast, so in English we translate it naturally as:

• The cast was announced today.

Turkish simply does not need a separate word for the.

Because it is the subject of the sentence, not the direct object.

In this sentence:

• Oyuncu kadrosu = the thing being talked about, the subject
• bugün = today
• açıklandı = was announced

Since the sentence is passive, the thing that would be the object in an active sentence becomes the subject.

Compare:

Active

• Birileri oyuncu kadrosunu bugün açıkladı.
• Someone announced the cast today.

Here oyuncu kadrosunu is the direct object, so it takes the accusative.

Passive

• Oyuncu kadrosu bugün açıklandı.
• The cast was announced today.

Now it is the subject, so there is no accusative ending.

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

The sentence you have:

• Oyuncu kadrosu bugün açıklandı.

is a very natural, neutral way to say it.

You could also say:

• Bugün oyuncu kadrosu açıklandı.
• Oyuncu kadrosu açıklandı bugün.

These are all possible, but the emphasis changes a little.

Very roughly:

• Oyuncu kadrosu bugün açıklandı.
  Neutral; focus often falls naturally on the verb.
• Bugün oyuncu kadrosu açıklandı.
  Slightly more emphasis on today.
• Oyuncu kadrosu açıklandı bugün.
  More marked; can sound more conversational or emphatic.

In Turkish, the element right before the verb often gets special focus.

Not always. It depends on context.

• oyuncu can mean actor in entertainment contexts
• it can also mean player in sports or games

And kadro can mean:

• cast
• lineup
• squad
• staff list

So oyuncu kadrosu could mean:

• cast for a film, series, play, etc.
• player lineup / squad in another context

You understand the intended meaning from the situation.

You could translate it that way literally, but it would usually sound less natural in English.

A more natural English translation is usually:

• The cast was announced today.

That is because oyuncu kadrosu is a set expression that often corresponds to cast in English, not a word-for-word combination.

This is a good example of why Turkish compounds are often best learned as whole phrases, not just as separate dictionary words.

An active version would include the person or organization that made the announcement.

For example:

• Yapımcı oyuncu kadrosunu bugün açıkladı. = The producer announced the cast today.

Notice the changes:

• oyuncu kadrosu → oyuncu kadrosunu
  because it becomes the direct object
• açıklandı → açıkladı
  because the verb becomes active

So passive and active versions are closely related:

• Oyuncu kadrosu bugün açıklandı. = The cast was announced today.
• X oyuncu kadrosunu bugün açıkladı. = X announced the cast today.

A simple pronunciation guide is:

• Oyuncu → oy-oon-joo
• kadrosu → kah-dro-soo
• bugün → boo-GUEN or roughly boo-goon for English speakers, though the ü sound is different from English u
• açıklandı → uh-chuhk-LAHN-duh

A few useful pronunciation notes:

• c in Turkish is like English j in jam
• ç is like ch
• ş is like sh
• ı is a vowel that does not exist in English; it is not the same as i
• ü is also different from English u

The stress in Turkish often falls toward the end, but not always perfectly predictably. In normal speech, this sentence sounds smooth and straightforward.