Fren teli ve sibop ayrı değiştirildi.

Because değişti means changed in the sense that something changed by itself, while değiştirildi means was changed / was replaced by someone.

A useful breakdown is:

• değişmek = to change
• değiştirmek = to change something, to replace something
• değiştirilmek = to be changed, to be replaced
• değiştirildi = was changed / was replaced

So in a repair or service context, değiştirildi is the natural choice if a mechanic or technician replaced the parts.

It can be broken down like this:

• değiş- = change
• -tir- = makes it causative/transitive, so değiştir- = change/replace something
• -il- = passive
• -di = past tense

So değiştirildi literally means something like it was caused to be changed, which in normal English becomes it was changed or it was replaced.

Turkish often uses the passive when the person doing the action is unknown, unimportant, or obvious from context.

In a sentence about repairs, the important information is usually the part that was replaced, not who replaced it. So:

• Fren teli ve sibop ayrı değiştirildi. = The brake cable and valve were replaced separately.

If you wanted an active version, you could say something like:

• Usta fren telini ve sibobu ayrı değiştirdi.
  The mechanic replaced the brake cable and the valve separately.

The passive sounds very natural in technical, service, and repair language.

Because Turkish often keeps the verb singular, especially with:

• inanimate things
• passive constructions
• general statements

So değiştirildi is perfectly normal even though there are two items.

English usually says were replaced, but Turkish does not need a plural verb here.

A plural form like değiştirildiler is generally not the normal choice in this kind of sentence.

Here ayrı means separately or individually.

Although ayrı often means separate as an adjective, it can also work adverbially in sentences like this, modifying the verb:

• ayrı değiştirildi = was replaced separately

So it tells you that the brake cable and the valve were not replaced together as one combined job/item, but separately.

Often, yes. Ayrı ayrı is a very common way to say separately / one by one / individually.

So you may also hear:

• Fren teli ve sibop ayrı ayrı değiştirildi.

That sounds a bit more explicit. But ayrı değiştirildi is also understandable and natural, especially in short technical or repair-style language.

Very roughly:

• ayrı = separately
• ayrı ayrı = separately, each one on its own

Fren teli is a noun compound.

It comes from:

• fren = brake
• tel = wire
• teli = wire + compound ending

In Turkish, many noun compounds are formed like this:

• first noun: bare form
• second noun: usually takes a 3rd-person possessive-type ending

So:

• fren teli = brake cable / brake wire

Other similar patterns:

• araba kapısı = car door
• telefon numarası = phone number

So the -i here is not an object ending. It is part of the noun compound.

Because sibop is just a simple noun by itself, while fren teli is a compound expression.

Compare:

• sibop = valve
• fren teli = brake cable

Only the compound noun has that built-in compound ending on the second word. Sibop does not need one because it is not part of a compound here.

Because in this sentence they are not direct objects. In the passive sentence, they function as the grammatical subject.

Compare:

Passive

• Fren teli ve sibop ayrı değiştirildi.
  The brake cable and valve were replaced separately.

Active

• Fren teli ve sibop ayrı değiştirildi. is passive, so no object marking.
• If you make it active and definite, you would usually get:
  • Fren telini ve sibobu ayrı değiştirdiler.
    They replaced the brake cable and the valve separately.

There you can see:

• fren teli → fren telini
• sibop → sibobu

So the lack of accusative here is because the sentence is passive.

Because Turkish normally puts adverbs before the verb, and the verb usually comes at the end of the sentence.

So this order is very typical:

• Fren teli ve sibop = subject
• ayrı = adverb
• değiştirildi = verb

This gives the standard Turkish sentence pattern:

• Subject + adverb + verb

Sibop is a common Turkish word in mechanical contexts, and it means valve.

It is often used for things related to:

• tires
• engines
• mechanical parts
• bicycles or motorcycles

So in everyday non-technical speech, learners may not see it often at first, but it is very normal in repair and equipment contexts.

Yes, depending on context, ayrı can suggest that they were handled separately rather than together. That may mean:

• as separate parts
• in separate procedures
• possibly at different times

But the exact nuance depends on the larger context. Without extra context, the safest understanding is simply:

• They were replaced separately, not as one combined replacement.

No. Turkish does not have words exactly like English the and a/an.

So Fren teli ve sibop ayrı değiştirildi can be understood from context as:

• the brake cable and the valve were replaced separately or
• a brake cable and a valve were replaced separately

In real use, the context usually makes it clear whether the speaker means specific known items. In a repair note or service report, English usually translates this with the.