Yarışmacı bilgi yarışmasını kazandı.

Because Turkish normally uses Subject–Object–Verb order.

So this sentence is:

• Yarışmacı = the contestant
• bilgi yarışmasını = the quiz / knowledge contest
• kazandı = won

Literally, the order is:

• The contestant the quiz won

That is completely normal in Turkish.
English usually puts the verb earlier, but Turkish typically saves it for the end.

Turkish word order can change for emphasis, but verb-final is the most neutral pattern.

Yarışmacı means contestant, competitor, or participant in a competition.

It comes from the root related to yarışmak, meaning to compete / race.

A useful breakdown is:

• yarış- = compete / race
• -ma = forms a noun like competition
• -cı / -ci / -cu / -cü = a common suffix meaning person associated with something

So yarışmacı is basically a person who competes.

Because they are different words built from the same root.

• yarışmacı = contestant
• yarışma = competition, contest
• bilgi yarışması = knowledge competition, quiz show, quiz contest

So the sentence uses one word for the person competing and another for the competition itself.

This kind of repetition is very natural in Turkish and does not sound strange.

Bilgi yarışması is a set expression meaning quiz competition, quiz show, or knowledge contest.

It is made of:

• bilgi = knowledge, information
• yarışması = its competition

Literally, it looks like knowledge competition, and that is how Turkish commonly says quiz or quiz show.

So:

• yarışma = contest / competition
• bilgi yarışması = quiz contest / knowledge competition

This is a very common Turkish noun compound pattern.

In bilgi yarışması, the -sı is part of a noun compound.

Turkish often forms compounds like this:

• X + Y-si/sı/su/sü

Examples:

• okul kapısı = school door
• araba kapısı = car door
• bilgi yarışması = knowledge competition

So yarışması here is not really saying his/her competition in the usual sense.
It is part of the standard compound structure.

This is one reason Turkish compounds can look a little like possessive forms to English speakers.

Because it is the direct object of the verb kazandı and it is specific/definite.

Turkish often marks a specific direct object with the accusative case.

The base form is:

• bilgi yarışması

As the definite direct object, it becomes:

• bilgi yarışmasını

So the sentence is talking about the quiz/contest, not just a quiz in a vague sense.

This is a very common question.

The word already ends in the compound/possessive-like suffix -sı:

• yarışması

When Turkish adds the accusative after this kind of ending, it usually inserts n before the case ending:

• yarışması
  • -ı → yarışmasını

So the n is a linking consonant.

That is why you get:

• bilgi yarışması → bilgi yarışmasını

not just bilgi yarışmasıı or something similar.

Kazandı is simple past tense: won.

Breakdown:

• kazan- = win
• -dı / -di / -du / -dü = past tense suffix, with vowel harmony

So:

• kazandı = won

This is the normal past tense used for completed actions.

Kazanmak most commonly means to win, but it can also mean to earn or to gain, depending on context.

Examples:

• Maçı kazandı. = He/She won the match.
• Para kazandı. = He/She earned money.
• Deneyim kazandı. = He/She gained experience.

In your sentence, because the object is bilgi yarışmasını, the meaning is clearly won the quiz/contest.

Turkish has no articles like the or a/an.

So yarışmacı can mean:

• the contestant
• a contestant

Which one is intended depends on context.

In an isolated example sentence, English often translates it as the contestant because that sounds natural. But Turkish itself does not use a separate word for the.

Very often, yes.

The accusative in Turkish usually suggests that the direct object is specific or definite.

Compare the idea:

• bilgi yarışması kazandı → not natural here, and would not express a normal definite object
• bilgi yarışmasını kazandı → won the specific quiz/contest

So the accusative helps signal that this is a known or specific contest.

Yes. Depending on context, bilgi yarışması can be translated as:

• quiz
• quiz contest
• quiz show
• knowledge competition

If this is about a TV program, quiz show may be the best translation.
If it is a school or live event, quiz contest or knowledge competition may fit better.

A few Turkish letters here are especially important:

• ş = sh
• c = j as in jam
• ı = the Turkish dotless i, a vowel that does not exist exactly in English

So roughly:

• yarışmacı ≈ yah-rush-ma-juh
• yarışmasını ≈ yah-rush-ma-suh-nuh

A few notes:

• a is usually like a in father
• ı is a relaxed, central vowel; do not pronounce it like English ee
• Turkish pronunciation is usually very regular and phonetic

In this sentence, yarışmacı is singular: contestant.

If it were plural, you would normally see:

• yarışmacılar = contestants

So:

• Yarışmacı bilgi yarışmasını kazandı. = The contestant won the quiz.
• Yarışmacılar bilgi yarışmasını kazandı. = The contestants won the quiz.

Turkish does not need an article, but it does usually show plural clearly when needed.

Yes, but the meaning or emphasis may shift.

The neutral order is:

• Yarışmacı bilgi yarışmasını kazandı.

You might also see:

• Bilgi yarışmasını yarışmacı kazandı.
  This puts more emphasis on the contestant as the one who won it.

• Kazandı yarışmacı bilgi yarışmasını.
  This is much less neutral and would sound marked or literary/dramatic in most contexts.

So yes, Turkish word order is flexible, but the given sentence is the most straightforward version.

The past tense suffix changes by vowel harmony.

The suffix has several forms:

• -dı
• -di
• -du
• -dü

The vowel is chosen to match the previous vowel in the stem.

Since the last vowel in kazan- is a, the correct form is:

• kazandı

This is a basic and very important pattern in Turkish.

Formally, it looks like one, but functionally it is usually best understood as a noun compound.

English speakers often notice that yarışması resembles his/her competition, but in a phrase like bilgi yarışması, it does not usually mean literal possession.

Instead, it works like:

• school bus
• computer screen
• tennis match

So bilgi yarışması is simply the standard Turkish way to say knowledge contest / quiz competition.

A very literal translation would be:

• The contestant won the knowledge competition.

A more natural English translation might be:

• The contestant won the quiz.
• The contestant won the quiz competition.
• The contestant won the quiz show.

The best choice depends on context.