Bu belgenin sahte olduğu kanıtlandı.

Because belgenin is the subject of the embedded clause sahte olduğu.

In Turkish, when you turn a clause into a noun-like phrase with -DIK forms such as olduğu, the subject of that clause is usually marked with the genitive:

• bu belge = this document
• bu belgenin sahte olduğu = that this document is fake

So:

• belge = document
• belgenin = of the document / the document’s

Here it does not literally mean possession in English. It is part of a very common Turkish pattern:

• X-in Y olduğu = the fact that X is Y

Olduğu comes from olmak = to be / to become.

In this sentence, sahte olduğu means:

• that it is fake
• its being fake
• the fact that it is fake

So olduğu is not just a simple past form of olmak. It is a nominalized form, meaning it turns the clause into something noun-like.

You can think of:

• sahte = fake
• sahte olduğu = being fake / that it is fake

It is built like this:

• ol- = stem of olmak
• -duk / -dık / -dik / -dük = a nominalizing suffix
• -u = third-person possessive ending

So historically:

• ol + duk + u
• which becomes olduğu

The k changes to ğ before a vowel, which is very common in Turkish.

That is why you get:

• olduğu not
• olduku

Because Turkish usually does not need a separate word like English that in this kind of sentence.

English says:

• It was proven that this document is fake.

Turkish instead builds a noun-like clause:

• Bu belgenin sahte olduğu = that this document is fake
• kanıtlandı = was proven

So the idea of that is carried by the grammatical structure, especially the -DIK form in olduğu, rather than by a separate word.

Because Turkish normally puts the main verb at the end of the sentence.

The structure is:

• Bu belgenin sahte olduğu = the whole content clause
• kanıtlandı = was proven

So the sentence is literally structured more like:

• This document’s being fake was proven.

That is normal Turkish word order.

Yes. Kanıtlandı is the passive past form of kanıtlamak = to prove.

Compare:

• Kanıtladılar. = They proved it.
• Kanıtlandı. = It was proven.

In your sentence, the thing that was proven is:

• Bu belgenin sahte olduğu = that this document is fake

So the whole sentence means:

• The fact that this document is fake was proven.

The person or people who proved it are not mentioned.

Because in this sentence, the clause bu belgenin sahte olduğu is the subject of kanıtlandı, not the direct object.

Compare these two patterns:

1. Bu belgenin sahte olduğu kanıtlandı.
   = It was proven that this document is fake.
   Here the clause is the subject, so it stays as olduğu.

2. Bu belgenin sahte olduğunu kanıtladılar.
   = They proved that this document is fake.
   Here the clause is the object of kanıtladılar, so it takes the accusative: olduğunu.

So:

• olduğu = subject form
• olduğunu = object form

This is a very common contrast in Turkish.

Not exactly.

This is one of the biggest points that confuses learners. In olduğu, the -DIK part is mainly doing a nominalizing job, not simply marking past tense the way English does.

So sahte olduğu does not necessarily mean only that it was fake. Depending on context, it can mean:

• that it is fake
• that it was fake
• that it has been fake

The main tense of the whole sentence is carried by kanıtlandı:

• kanıtlandı = was proven

That is why the full sentence is naturally translated as:

• It was proven that this document is fake.

Because sahte is the predicate adjective inside the embedded clause.

In a simple sentence:

• Bu belge sahte. = This document is fake.

When Turkish turns that idea into a noun-like clause, it often uses olmak:

• bu belgenin sahte olduğu

So sahte itself does not need an extra ending here. The grammatical work is being done by olduğu.

The pattern is:

• [Genitive subject] + [predicate + olduğu] + [main verb]

In your sentence:

• Bu belgenin = genitive subject of the embedded clause
• sahte olduğu = that it is fake
• kanıtlandı = was proven

This pattern is extremely common in Turkish. Similar examples:

• Onun haklı olduğu anlaşıldı. = It became clear that he was right.
• Bu işin zor olduğu biliniyor. = It is known that this job is difficult.
• Toplantının iptal olduğu duyuruldu. = It was announced that the meeting was canceled.

So this sentence is a very standard Turkish way to say that... clauses.

Yes, absolutely. That is actually very close to the Turkish structure.

A more literal breakdown is:

• Bu belgenin sahte olduğu = the fact that this document is fake
• kanıtlandı = was proven

Natural English usually says:

• It was proven that this document is fake.

But if you want to understand the Turkish grammar, The fact that this document is fake was proven is very helpful.

Yes. A few close alternatives are:

• Bu belgenin sahte olduğu ispatlandı.
  İspatlandı also means was proven.

• Bu belgenin sahte olduğu ortaya çıktı.
  = It turned out that this document is fake.

• Bu belgenin sahte olduğu kanıtlanmış oldu.
  = roughly It ended up being proven that this document is fake.

But Bu belgenin sahte olduğu kanıtlandı is a very normal, standard sentence.

Because that would not express the same structure naturally.

• Bu belge sahte. = This document is fake.
• Bu belgenin sahte olduğu kanıtlandı. = It was proven that this document is fake.

Turkish usually needs the nominalized clause sahte olduğu to represent the whole proposition that the document is fake.

So the sentence is not about the document alone; it is about the claim/fact that the document is fake. That is why the longer structure is used.