Kuşkulu belgeyi neden kabul ettin?

Because belgeyi is the definite direct object of the sentence.

• belge = document
• belgeyi = the document / the document as the object

In Turkish, a specific or definite direct object usually takes the accusative ending. Here the accusative is -(y)i.

Why -yi and not just -i?

• belge ends in a vowel
• Turkish inserts a buffer consonant y before a vowel-initial suffix

So:

• belge + i → belgeyi

That is why kuşkulu belgeyi means the suspicious document, not just a suspicious document.

Kuşkulu means suspicious, doubtful, or questionable, depending on context.

It comes from:

• kuşku = doubt, suspicion
• -lu/-lü = a suffix meaning with, having, or characterized by

So kuşkulu literally has the sense of having suspicion/doubt or causing suspicion.

In this sentence, kuşkulu belge means something like:

• a suspicious document
• a questionable document
• a document that raises doubt

Because Turkish often drops subject pronouns when they are already clear from the verb ending.

Here the verb is ettin, and the ending tells you the subject is you singular.

• ettim = I accepted
• ettin = you accepted
• etti = he/she/it accepted
• ettik = we accepted

So Turkish does not need sen here.
If you added sen, it would usually give extra emphasis:

• Sen kuşkulu belgeyi neden kabul ettin? = Why did you accept the suspicious document?

Kabul ettin comes from the compound verb kabul etmek, which means to accept.

Breakdown:

• kabul = acceptance / acceptance as a noun element
• etmek = to do, to make
  Together: kabul etmek = to accept

In this sentence:

• et- = verb stem
• -ti = past tense marker here
• -n = you singular

So:

• kabul et- + -ti + -n → kabul ettin

Meaning: you accepted

Because of how the verb stem and the past tense suffix come together.

The verb part is et-.
The past tense suffix is basically -di, but after a voiceless consonant like t, it becomes -ti.

So:

• et + ti + n → ettin

The two t sounds meet, so you see tt in writing.

This is completely normal in Turkish:

• git- + ti → gitti
• et- + ti → etti
• ret et- + ti → reddetti is a more complicated spelling case, but the same kind of sound interaction happens in many verbs

Because kabul etmek is a very common Turkish compound verb made from:

• a noun or borrowed lexical item: kabul
• the light verb etmek

These are normally written as separate words:

• yardım etmek = to help
• fark etmek = to notice
• kabul etmek = to accept

So kabul ettin is the normal spelling.

Because this is a wh-question, not a yes/no question.

The word neden already means why, so that by itself makes the sentence a question:

• Kuşkulu belgeyi neden kabul ettin? = Why did you accept the suspicious document?

You use mi for yes/no questions:

• Kuşkulu belgeyi kabul ettin mi? = Did you accept the suspicious document?

So:

• neden = why-question
• mi = yes/no question marker

You normally do not use both together in this kind of sentence.

Neden means why.

Yes, it is very close in meaning to:

• niye = why
• niçin = why / for what reason

All three can often be used in similar situations. Very roughly:

• neden can sound a bit more neutral or standard
• niye can sound a bit more conversational
• niçin can sound slightly more formal or literary in some contexts

So these are all possible:

• Kuşkulu belgeyi neden kabul ettin?
• Kuşkulu belgeyi niye kabul ettin?
• Kuşkulu belgeyi niçin kabul ettin?

The meaning stays essentially the same.

It is the simple past (also often called the definite past in Turkish grammar descriptions).

So ettin means:

• you accepted
• you did accept

This tense is used for a completed action that the speaker treats as a known fact.

In this sentence, the speaker is not asking whether you accepted it.
The speaker assumes that you did, and asks for the reason:

• not Did you accept it?
• but Why did you accept it?

No. Turkish word order is fairly flexible, although some orders sound more neutral than others.

The sentence:

• Kuşkulu belgeyi neden kabul ettin?

is a natural order. But Turkish can move parts around for focus or emphasis.

For example:

• Neden kuşkulu belgeyi kabul ettin?
• Kuşkulu belgeyi kabul ettin, neden?
• Sen kuşkulu belgeyi neden kabul ettin?

These can all work in the right context, but they may shift what sounds emphasized.

A useful basic idea:

• Turkish often places the most important or focused information near the verb
• but this is not as rigid as English word order

Because Turkish normally places the object before the verb.

A very common Turkish pattern is:

• Subject + Object + Verb

In this sentence, the direct object is:

• kuşkulu belgeyi = the suspicious document

Then the verb comes at the end:

• kabul ettin = you accepted

That is why the sentence ends with the verb phrase, which is very typical in Turkish.

Not naturally in this form.

Because of the accusative ending -yi, belgeyi strongly suggests a specific document:

• the suspicious document
• that suspicious document
• a certain suspicious document in context

If you wanted a more indefinite meaning like a suspicious document, Turkish would usually avoid the accusative:

• Neden kuşkulu bir belge kabul ettin? = Why did you accept a suspicious document?

So the difference is important:

• kuşkulu belgeyi = a specific suspicious document
• kuşkulu bir belge = a nonspecific suspicious document

A rough pronunciation guide is:

• Kuşkulu → koosh-koo-loo
• belgeyi → bel-ge-yi
• neden → ne-den
• kabul → ka-bool
• ettin → et-tin

A few useful sound notes:

• ş sounds like English sh
• u is like oo in food
• e is usually a clear eh sound
• i is like ee in machine, but shorter

So the full sentence is roughly:

koosh-koo-loo bel-ge-yi ne-den ka-bool et-tin?

Not perfect IPA, but often helpful for an English speaker starting out.