Otobüs hızını azalttı.

A simple breakdown is:

• otobüs = bus
• hızını = its speed / the speed as the direct object
• azalttı = reduced / decreased

More exactly:

• hız = speed
• -ı = 3rd person possessive suffix, so hızı = its speed
• -n- = a linking consonant
• -ı = accusative case, marking the direct object

So hızını literally means its speed as the thing being affected by the verb.

And:

• azalt- = reduce, decrease something
• -tı = past tense

So the whole sentence is literally The bus reduced its speed.

Turkish does not have a word exactly like English the.

So otobüs can mean:

• a bus
• the bus

The context tells you which one is meant. In this sentence, English will usually use the bus, because it sounds like a specific bus being talked about.

Because Turkish usually marks extra information directly on the noun.

Here, hızını shows two important things:

• whose speed it is: its speed
• that this noun is the direct object of the verb

So:

• hız = speed
• hızı = its speed
• hızını = its speed as the object

If you only said hız, that would just mean speed in a more general, unmarked way, and it would not fit this sentence as naturally.

That n is a buffer or linking consonant.

The structure is:

• hız
  • -ı = hızı (its speed)
• hızı
  • -ı (accusative) needs a linker
• so it becomes hızını

This linking n often appears when a noun with a possessive suffix takes another ending after it.

So the n is not a separate word or meaning by itself; it is just there to connect the suffixes smoothly.

Because it is the direct object of the verb azalttı.

The bus is affecting a specific thing: its speed. In Turkish, specific direct objects are usually marked with the accusative.

So here:

• otobüs = subject
• hızını = definite/specific direct object
• azalttı = verb

That is why the object is marked.

This is a very common question.

Otobüs hızını azalttı already means The bus reduced its speed. Since the subject and the possessor are the same thing, Turkish normally just shows that possession on the object itself: hızını = its speed.

If you said otobüsün hızını azalttı, that would usually suggest a different subject, often one that is omitted:

• Otobüsün hızını azalttı. = He/She/It reduced the bus’s speed.

So:

• Otobüs hızını azalttı = The bus reduced its speed
• Şoför otobüsün hızını azalttı = The driver reduced the bus’s speed

That is the key difference.

Because azalmak and azaltmak are different verbs.

• azalmak = to decrease, to become less
  This is intransitive: the thing decreases by itself.
• azaltmak = to decrease something, to reduce something
  This is transitive: something causes a reduction.

So:

• Otobüsün hızı azaldı. = The bus’s speed decreased.
• Otobüs hızını azalttı. = The bus reduced its speed.

In English those can be quite close, but Turkish clearly separates them.

It comes from the verb azaltmak.

You can think of it like this:

• azal- = decrease
• -t- = a causative element, making it reduce something
• azalt- = reduce
• -tı = past tense

So azalttı means reduced.

The double tt happens because the verb stem already ends in t, and the past tense suffix appears here as -tı.

Because Turkish is normally an SOV language:

• Subject
• Object
• Verb

So this sentence follows the common pattern:

• Otobüs = subject
• hızını = object
• azalttı = verb

English usually uses SVO:

• The bus
• reduced
• its speed

Turkish can move parts around for emphasis, but putting the verb last is the most neutral and standard order.

Yes, it is natural and correct.

But depending on context, Turkish speakers might also say:

• Otobüs yavaşladı. = The bus slowed down.
• Otobüsün hızı azaldı. = The bus’s speed decreased.

These are slightly different in focus:

• Otobüs hızını azalttı focuses on the bus reducing its speed
• Otobüs yavaşladı is a simpler everyday way to say the bus slowed down
• Otobüsün hızı azaldı focuses on the speed itself decreasing

So your sentence is good; it is just one way of expressing the idea.

A few pronunciation points matter here:

• ı is the Turkish dotless i
• it is not the same as English ee in see
• it is also not the same as Turkish i

Approximate pronunciations:

• hızını ≈ huh-zuh-nuh, but with a back vowel for each ı
• azalttı ≈ ah-zahl-tuh

A few specific sounds:

• z sounds like English z
• double tt is pronounced clearly
• final ı in azalttı is again the dotless ı

Also, otobüs has ü, which is pronounced like the German ü or the French u, not like English oo.