Fotovoltaik modüller montaj rayına yerleştirildi.

A simple breakdown is:

• fotovoltaik = photovoltaic
• modüller = modules
• montaj rayına = onto / to the mounting rail
• yerleştirildi = was/were placed

So the structure is basically:

• subject: fotovoltaik modüller
• destination/location aimed at: montaj rayına
• verb: yerleştirildi

Turkish usually puts the verb at the end, so this is a normal sentence order.

Because Turkish often forms noun + noun combinations with a special compound pattern.

Here:

• montaj = assembly / mounting
• ray = rail
• montaj rayı = mounting rail

That -ı on rayı is part of the compound structure. It is very common in Turkish:

• kapı kolu = door handle
• araba kapısı = car door
• montaj rayı = mounting rail

So montaj rayı is the normal way to say mounting rail.

Because the noun is not just sitting there by itself; it has the dative case ending.

• montaj rayı = the mounting rail
• montaj rayına = to / onto the mounting rail

The verb yerleştirmek often uses the dative when something is being placed somewhere, because it marks the destination.

So:

• montaj rayına yerleştirildi = was placed onto the mounting rail

Because Turkish and English package this idea differently.

With a verb of placement like yerleştirmek, Turkish usually focuses on the target location and uses the dative:

• rayına = to / onto the rail

English often says on the rail, but Turkish prefers the idea of movement toward the place.

A useful contrast is:

• montaj rayına yerleştirildi = it was placed onto the mounting rail
• montaj rayında = it is on the mounting rail

So:

• -a / -e = destination, toward, onto
• -da / -de = location, on/in/at

That n is a buffer consonant.

Since montaj rayı already ends in a possessive/compound-type ending -ı, Turkish normally inserts n before certain case endings.

So:

• montaj rayı
• montaj rayı + a
• becomes montaj rayına

This same thing happens in many similar forms:

• kapısı → kapısına
• adı → adına

So the n is there to connect the word smoothly to the case ending.

Yerleştirildi can be understood as:

• yerleştir- = place, install, set
• -il- = passive
• -di = past tense

So yerleştirildi means:

• was placed
• were placed

It is a passive past form.

That is why the sentence does not say who did the placing. The focus is on the modules, not on the installer.

Because Turkish usually packs that meaning into the verb itself.

In English:

• were placed

In Turkish:

• yerleştirildi

The ideas of:

• past tense
• passive voice

are expressed by suffixes attached to the verb, not by adding a separate helping verb like were.

So one Turkish verb can correspond to two English words.

Because in Turkish, a plural subject does not always require a plural verb.

This is especially normal with inanimate things like modules.

So:

• Fotovoltaik modüller yerleştirildi = normal
• a plural-marked verb here would sound unnecessary or unnatural in standard usage

Turkish often leaves the verb in the default third-person singular form when the subject is already clearly plural.

Because in this sentence it is the subject, not the direct object.

In an active sentence, you might have something like:

• İşçiler fotovoltaik modülleri montaj rayına yerleştirdi.
• The workers placed the photovoltaic modules onto the mounting rail.

There, modülleri is the direct object, so it takes the accusative ending.

But in your sentence, the passive changes the structure:

• Fotovoltaik modüller ... yerleştirildi.
• The photovoltaic modules were placed ...

Now fotovoltaik modüller is the grammatical subject, so there is no accusative ending.

Because Turkish does not have a direct equivalent of the English definite article the.

Definiteness is often understood from:

• context
• word order
• case marking
• the general situation

So fotovoltaik modüller can mean:

• photovoltaic modules
• the photovoltaic modules

depending on context.

If Turkish wants to make something explicitly indefinite, it can use bir:

• bir modül = a module

But there is no separate everyday word that works exactly like English the.

The most neutral Turkish order is to put the verb at the end, as here:

• Fotovoltaik modüller montaj rayına yerleştirildi.

But Turkish word order is fairly flexible, especially for emphasis.

For example, you could move things around to emphasize the location:

• Montaj rayına fotovoltaik modüller yerleştirildi.

This still means essentially the same thing, but the emphasis shifts a bit.

What usually stays true is that the verb commonly comes at the end in normal written Turkish.

Usually, no if you mean placed onto the rail.

• rayına = onto / to the rail
• rayında = on the rail

With yerleştirmek, Turkish normally uses the dative because the verb describes putting something into a position.

So:

• montaj rayına yerleştirildi = it was placed onto the mounting rail

If you say rayında, that sounds more like describing where something is located, not the destination of the placing action.