Alyan anahtarı sele borusunda kullanıldı.

A word-by-word breakdown is:

• Alyan anahtarı = Allen key / hex key
• sele borusunda = on/in/at the seat tube
• kullanıldı = was used

So the structure is basically:

• Alyan anahtarı → the tool being talked about
• sele borusunda → where it was used
• kullanıldı → the action, in the passive: was used

Because Alyan anahtarı is a very common Turkish noun compound.

• anahtar = key
• alyan anahtarı = Allen key

In Turkish, compounds like this often work as:

• first noun: bare form
• second noun: takes the 3rd person possessive ending

So:

• kapı kolu = door handle
• araba kapısı = car door
• alyan anahtarı = Allen key

That -ı in anahtarı is not the accusative here. It is part of the compound.

Sele borusu means seat tube as a noun phrase by itself.

To say in/on/at the seat tube, Turkish adds the locative suffix:

• sele borusu = seat tube
• sele borusunda = in/on/at the seat tube

The reason it becomes -nda rather than just -da is that borusu already contains the compound ending -su. Then the locative is added after that.

So the layers are:

• boru = tube
• sele borusu = seat tube
• sele borusunda = on/in the seat tube

It can correspond to in, on, or at in English depending on context.

The Turkish locative suffix -DA is broader than any single English preposition. It often covers several English choices.

So sele borusunda could be translated naturally as:

• in the seat tube
• on the seat tube
• at the seat tube

The best English choice depends on the mechanical context, not just the grammar.

Because the sentence is saying that the Allen key was used, without focusing on who used it.

• kullanmak = to use
• kullanıldı = was used

This is a very normal way to speak in Turkish, especially in technical, instructional, or descriptive contexts.

Compare:

• Birisi alyan anahtarını sele borusunda kullandı. = Someone used the Allen key on the seat tube.
• Alyan anahtarı sele borusunda kullanıldı. = The Allen key was used on the seat tube.

The second version is more impersonal and sounds more like a report or instruction manual.

It comes from the verb kullanmak (to use).

A simplified breakdown is:

• kullan- = use
• -ıl- = passive marker
• -dı = past tense

So:

• kullanıldı = was used

Because of vowel harmony and sound changes, the passive marker appears here as -ıl-.

Because in this sentence it is effectively the subject of a passive sentence, not the direct object of an active one.

Compare:

• Usta alyan anahtarını kullandı. = The mechanic used the Allen key.
  Here anahtarını is the direct object, so it takes the accusative.

• Alyan anahtarı kullanıldı. = The Allen key was used.
  Here alyan anahtarı is the thing being talked about as the subject of the passive sentence, so there is no accusative.

This is why anahtarı here is just the compound form, not an accusative object form.

The sentence follows the very common Turkish pattern:

subject/topic + place + verb

So:

• Alyan anahtarı = subject/topic
• sele borusunda = place/location
• kullanıldı = verb

Turkish normally puts the verb at the end, so this word order is very natural.

Yes. Turkish word order is flexible, though the default version is usually the most neutral.

For example:

• Sele borusunda alyan anahtarı kullanıldı.

This still means essentially the same thing, but now sele borusunda is brought earlier, which can give slightly more emphasis to the location.

The original sentence is a neutral, straightforward version.

No, not really in normal usage. In modern Turkish, alyan is commonly used as a regular noun meaning Allen key / hex key, even though historically it comes from a brand/name.

So in the middle of a sentence, you would usually write it lowercase:

• alyan anahtarı

It is capitalized here only because it is the first word of the sentence.

A strong clue is that the second noun carries a possessive-looking ending:

• anahtar → anahtarı
• boru → borusu

This is very common in Turkish noun compounds. The pattern often looks like:

• X Y-si / Y-sı / Y-su / Y-sü
• or X Y-ı / Y-i / Y-u / Y-ü

Examples:

• kahve fincanı = coffee cup
• mutfak masası = kitchen table
• bisiklet selesi = bicycle saddle
• sele borusu = seat tube

So when you see two nouns together and the second has this ending, it is often a compound.

An active version would need a subject, for example:

• Tamirci alyan anahtarını sele borusunda kullandı. = The repairer used the Allen key on the seat tube.

Notice the changes:

• Alyan anahtarı → alyan anahtarını
  because it becomes the direct object
• kullanıldı → kullandı
  because the sentence is no longer passive

So passive and active are built differently, even though the core meaning stays similar.