Montaj rayı çatı kancasıyla sabitlendi.

Because montaj rayı is a Turkish noun compound, meaning mounting rail.

In many Turkish noun compounds, the second noun takes the suffix -(s)I:

• çatı kancası = roof hook
• montaj rayı = mounting rail

So the -ı in rayı is part of the compound form, not something added because of the sentence structure.

No. That is a very common confusion.

In this sentence, rayı is not the accusative. It is the compound ending from montaj rayı.

Why we know this:

• The sentence means The mounting rail was secured...
• So montaj rayı is the thing being talked about, not a direct object in an active sentence.
• The verb sabitlendi is an intransitive/passive-type form meaning was secured / became secured

So here:

• montaj rayı = the mounting rail
  not
• montaj ray-ı = the rail (as a definite object)

It means with the roof hook or by means of the roof hook.

It breaks down like this:

• çatı = roof
• kancası = hook / its hook / hook of ..., but here part of the compound çatı kancası = roof hook
• -yla = with, a suffixed form of ile

So:

• çatı kancasıyla = with the roof hook

Turkish often attaches ile to the end of a word instead of writing it separately.

So these are equivalent:

• çatı kancası ile
• çatı kancasıyla

When ile attaches after a vowel, it usually becomes -yla or -yle.

Since kancası ends in a vowel, you get:

• kancası + ile → kancasıyla

This attached form is very common and natural.

Because çatı kancası is also a noun compound.

Just like montaj rayı, the second noun gets the compound suffix:

• çatı kancası = roof hook

Then ile is added:

• çatı kancası + ile
• çatı kancasıyla

So the -sı is not random; it is part of the compound çatı kancası.

sabitlendi means was secured, got fixed, or became fixed in place, depending on context.

A helpful way to see it is:

• sabit = fixed, stable
• sabitlemek = to fix, secure, fasten
• sabitlenmek = to become fixed / to be secured
• sabitlendi = became fixed / was secured

The ending -di is the past tense.

So sabitlendi is a past-tense form meaning that the securing happened.

Because Turkish often uses this kind of passive/intransitive form when the doer is unknown, unimportant, or obvious from context.

So instead of saying:

• Someone secured the mounting rail with the roof hook

Turkish naturally says:

• Montaj rayı çatı kancasıyla sabitlendi.
• The mounting rail was secured with the roof hook.

This is especially common in technical, instructional, and descriptive language.

Because Turkish normally places the finite verb at the end.

So the sentence follows a typical Turkish pattern:

• Montaj rayı = the mounting rail
• çatı kancasıyla = with the roof hook
• sabitlendi = was secured

This verb-final structure is one of the most basic word-order patterns in Turkish.

Yes, Turkish word order is flexible, but the emphasis changes.

For example:

• Montaj rayı çatı kancasıyla sabitlendi.
  Neutral: the mounting rail was secured with the roof hook.

• Çatı kancasıyla montaj rayı sabitlendi.
  Puts more emphasis on with the roof hook.

In normal statements, though, the verb usually still stays at the end.

Because Turkish does not have articles like English the and a/an.

Whether something is definite or indefinite is understood from:

• context
• word order
• case marking
• whether the noun is specific or general in the situation

So montaj rayı can be understood as the mounting rail here because the sentence sounds like it is referring to a specific part in a technical context.

Usually that is the best English translation here, but the Turkish can feel slightly broader.

Depending on context, sabitlendi can suggest:

• was secured
• was fastened
• was fixed in place
• became secured

In a technical sentence like this, was secured is the most natural English choice.