Arabanın hızı çok yüksek.

Because -ın here is the genitive ending, which often corresponds to English of or 's.

• araba = car
• arabanın = of the car / the car's

So arabanın hızı literally means the car's speed.

Also notice the extra n before the ending:

• araba
  • -ın → arabanın

That n helps connect the word smoothly to the suffix.

This is because Turkish uses a possessive suffix on the possessed noun in this kind of phrase.

So in arabanın hızı:

• arabanın = the car's
• hızı = its speed / the speed

This is a very common Turkish pattern:

• evin kapısı = the house's door / the door of the house
• kitabın adı = the book's name / the name of the book
• arabanın hızı = the car's speed

In other words, Turkish usually marks both parts of this structure:

1. the owner with the genitive
2. the owned thing with the possessive suffix

That is just how the standard Turkish genitive–possessive construction works.

English:

• the car's speed

Turkish:

• araba-nın hız-ı

So Turkish often shows the relationship twice:

• -ın on the possessor
• -ı on the possessed item

This makes the relationship very clear.

In Turkish, the verb to be is often not stated in the present tense, especially in simple third-person sentences.

So:

• Arabanın hızı çok yüksek. literally looks like
• The car's speed very high.

But it means:

• The car's speed is very high.

This is completely normal Turkish grammar.

Compare:

• Hava sıcak. = The weather is hot.
• Ev büyük. = The house is big.
• Arabanın hızı çok yüksek. = The car's speed is very high.

Turkish often puts the main predicate at the end. Here, the main idea is yüksek = high, so it comes last.

Structure:

• Arabanın hızı = the car's speed
• çok = very
• yüksek = high

So the sentence is built like:

[subject] + [degree word] + [predicate adjective]

That is why çok yüksek comes at the end.

Çok here means very.

It comes before the adjective it modifies, just as in English:

• çok yüksek = very high
• çok büyük = very big
• çok güzel = very beautiful

So in this sentence:

• yüksek = high
• çok yüksek = very high

Good question. Both can relate to speed, but they are used differently.

• yüksek = high
• hızlı = fast

This sentence talks about the speed itself, so yüksek fits naturally:

• Arabanın hızı çok yüksek. = The car's speed is very high.

If you want to describe the car directly, you would more naturally say:

• Araba çok hızlı. = The car is very fast.

So:

• hız = speed
• yüksek hız = high speed
• hızlı araba = fast car

Turkish does not usually have a separate word equivalent to English the.

Whether something is understood as a or the often depends on context.

So:

• araba can mean a car or the car
• arabanın hızı is usually understood as the car's speed in context

Turkish relies more on structure and context than on articles.

Because of vowel harmony, which is a major feature of Turkish.

The vowel in the suffix changes to match the vowels in the word.

Here:

• araba has back vowels, so the genitive becomes -ın
• hız also has a back vowel, so the possessive becomes -ı

That is why we get:

• arabanın
• hızı

If the word had different vowels, the suffix might look different:

• ev → evin
• göl → gölün
• yüz → yüzü

Yes, but it means something slightly different in structure.

• Arabanın hızı çok yüksek. = The car's speed is very high.
• Araba çok hızlı. = The car is very fast.

These are very close in meaning, but not identical grammatically.

The first focuses on speed as a noun. The second describes the car directly with an adjective.

Both are natural, but the sentence you were given is more formal or more explicitly about speed.

They are related, but not the same.

• arabanın hızı = the speed of the car / the car's speed

  • usually a specific car

• araba hızı = car speed

  • more general, more like a compound noun

For example:

• Arabanın hızı çok yüksek. = The speed of the car is very high.
• Araba hızı önemlidir. = Car speed is important.

So the version in your sentence refers to a specific car.

A simple pronunciation guide is:

• Arabanın ≈ ah-rah-bah-NUHN
• hızı ≈ huh-ZUH
• çok ≈ chok
• yüksek ≈ YOOK-sek

A few helpful notes:

• ı is not the English i sound. It is a more neutral vowel, somewhat like the vowel in roses for some speakers, but farther back.
• ç is like ch in chair
• ö and ü are front rounded vowels, which may be unfamiliar to English speakers

You do not need perfect pronunciation right away, but it helps to notice that ı and i are different sounds in Turkish.