Onun ölümü herkesi üzdü.

Because ölümü means his/her death or its death, not just death.

• ölüm = death
• ölüm-ü = his/her/its death

The ending -ü here is a 3rd person possessive suffix. It shows that the death belongs to him/her.

So:

• ölüm = death
• onun ölümü = his/her death

In Turkish, possession is often shown twice:

• the possessor: onun = his/her
• the possessed noun with a possessive ending: ölümü

This is a very common pattern in Turkish.

Onun is the genitive form of o.

• o = he / she / it
• onun = his / her / its

In this sentence, onun marks the possessor:

• onun ölümü = his/her death

So onun does not mean him/her here. It means his/her.

Yes, often it can be omitted if the meaning is clear from context.

• Onun ölümü herkesi üzdü.
• Ölümü herkesi üzdü.

Both can mean His/Her death upset everyone, but the version with onun is more explicit.

Turkish often drops pronouns when the meaning is already understood. However, keeping onun can help if you want to emphasize whose death you mean or avoid ambiguity.

It actually is written ölümü because the base word is ölüm.

Breakdown:

• ölüm = death
• ölüm + ü = ölümü

The -ü is chosen because of vowel harmony. The last vowel in ölüm is ü, so the possessive suffix appears as -ü.

This is not an accusative ending here. It is the possessive suffix meaning his/her/its.

Not in this sentence.

In ölümü, the -ü is the 3rd person possessive suffix:

• ölümü = his/her death

That said, Turkish can sometimes produce forms that look the same on the surface. For example, a possessed noun can also take an accusative ending in other contexts. But here, ölümü is simply the subject phrase his/her death.

The main clue is the structure:

• onun ölümü = his/her death

This is a possession structure, not a direct object.

Because herkesi is the direct object of the verb üzdü.

• herkes = everyone
• herkesi = everyone / everybody, as the object

Turkish uses the accusative case for specific direct objects. Since everyone here is a definite, specific object, Turkish marks it:

• herkesi üzdü = upset everyone

So:

• herkes = everyone
• herkesi = everyone (object form)

Üzdü comes from the verb üzmek, which means to upset, to sadden, or to make someone sad.

Breakdown:

• üz- = verb stem
• -dü = past tense

So:

• üzdü = upset / made sad

In this sentence:

• Onun ölümü herkesi üzdü.
• His/Her death upset everyone.

Notice that üzmek is a verb that usually takes an object:

• something or someone upsets someone

So here:

• onun ölümü = the thing doing the upsetting
• herkesi = the people affected

Because Turkish normally follows Subject–Object–Verb word order.

In this sentence:

• Onun ölümü = subject
• herkesi = object
• üzdü = verb

So the natural Turkish order is:

• Onun ölümü herkesi üzdü.

This is different from English, which is usually Subject–Verb–Object:

• His/Her death upset everyone.

Turkish word order can be flexible for emphasis, but putting the verb at the end is the most neutral pattern.

You can tell from both meaning and structure.

The thing that causes the action is onun ölümü:

• his/her death is what upset people

The noun phrase herkesi has the accusative ending -i, which marks it as the direct object. That leaves onun ölümü as the subject.

So the sentence works like this:

• Onun ölümü = subject
• herkesi = object
• üzdü = verb

It can mean all three, depending on context.

Turkish does not usually mark grammatical gender in pronouns:

• o = he / she / it
• onun = his / her / its

So onun ölümü could mean:

• his death
• her death
• its death

You need context to know which one is intended.

Yes, that is a natural English way to express the same idea.

The Turkish sentence is active in form:

• Onun ölümü herkesi üzdü.
• literally: His/Her death upset everyone.

But a more natural English translation could also be:

• Everyone was saddened by his/her death.

The Turkish meaning stays the same: the death caused sadness in everyone.

This is a very common question.

• üzmek = to upset someone, to make someone sad
• üzülmek = to become sad, to be upset, to feel sad

So:

• Onun ölümü herkesi üzdü. = His/Her death upset everyone.

But if you wanted to say Everyone was upset, you might use üzülmek:

• Herkes onun ölümüne üzüldü. = Everyone was upset/saddened by his/her death.

So üzmek is transitive and affects an object, while üzülmek describes the person experiencing sadness.

Because the grammar is different.

In:

• Onun ölümü herkesi üzdü. = his/her death is the subject

Here, ölümü is a possessed noun: his/her death.

But in:

• Herkes onun ölümüne üzüldü. = Everyone was saddened by his/her death

Here, ölümüne has the dative ending -e / -a (with buffer -n- after the possessive form), because üzülmek commonly goes with the dative:

• bir şeye üzülmek = to be sad about something

So:

• ölümü = his/her death
• ölümüne = to/about his/her death

Yes. Turkish word order is flexible, though the neutral order is:

• Onun ölümü herkesi üzdü.

Other orders are possible for emphasis, for example:

• Herkesi onun ölümü üzdü. This can emphasize everyone.
• Üzdü herkesi onun ölümü. This is more marked and less neutral.

A learner should usually stick with the standard order unless there is a clear reason to shift focus.

A rough English-friendly pronunciation would be:

• oh-NOON uh-LU-mu hair-keh-SEE UZ-du

A few notes:

• ö is like the vowel in German schön or French peu
• ü is like the vowel in German über or French tu
• r is lightly rolled or tapped
• stress in Turkish is not always strong in the English sense, but saying each syllable clearly helps

The actual syllables are:

• o-nun
• ö-lü-mü
• her-ke-si
• üz-dü

Here is a word-by-word breakdown:

• Onun
  = his/her/its
  = genitive form of o

• ölümü
  = death + 3rd person possessive = his/her/its death

• herkesi
  = everyone + accusative = everyone as the direct object

• üzdü
  = upset / made sad = past tense of üzmek

So the structure is:

• [Onun ölümü] [herkesi] [üzdü].
• [His/Her death] [everyone-ACC] [upset].

That is why the meaning is His/Her death upset everyone.