Bu asılsız söylentiyi kim uydurdu?

Because it is the direct object of the verb, and in Turkish a specific/definite direct object usually takes the accusative case.

• Base noun: söylenti = rumor
• Accusative form: söylentiyi = the rumor / this rumor (as the object)

So:

• Bu asılsız söylenti = this unfounded rumor
• Bu asılsız söylentiyi = this unfounded rumor (as the thing someone made up)

The -yi part is the accusative ending. It appears as -ı, -i, -u, -ü depending on vowel harmony, and sometimes a buffer consonant y is added after a vowel-final noun.

Here:

• söylenti ends in a vowel
• accusative is -i
• buffer y is inserted
• so: söylenti + y + i = söylentiyi

Bu means this.

Turkish does not have articles like English a/an/the. Instead, definiteness is often understood from context, demonstratives like bu, or case marking.

So:

• bu = this
• bu söylenti = this rumor
• bu söylentiyi = this rumor (as a specific object)

English needs the or this, but Turkish handles this differently.

Asılsız means baseless, groundless, or unfounded.

It comes from:

• asıl = origin, basis, essence
• -sız = without

So literally, asılsız means without basis.

This -sız / -siz / -suz / -süz suffix is very common in Turkish:

• tuz = salt → tuzsuz = unsalted / without salt
• umut = hope → umutsuz = hopeless
• ev = house/home → evsiz = homeless

So asılsız söylenti is literally something like a rumor without basis.

Turkish word order is often Subject–Object–Verb by default, and the verb usually comes at the end.

In this sentence:

• Bu asılsız söylentiyi = object
• kim = subject/question word
• uydurdu = verb

So the structure is:

• [Object] [Subject/Question word] [Verb]

This is very natural in Turkish.

English prefers:

• Who made up this unfounded rumor?

Turkish prefers:

• This unfounded rumor who made up?

That does not mean Turkish word order is totally fixed. Turkish is fairly flexible, and moving things around can change emphasis. But with neutral wording, final-position verbs are very common.

Kim means who.

Here it is asking about the person who did the action, so it functions like the subject of the sentence.

• Kim uydurdu? = Who made it up?
• Bu asılsız söylentiyi kim uydurdu? = Who made up this unfounded rumor?

Because kim is the subject here, it does not take extra case marking.

Compare:

• Kim geldi? = Who came?
• Kimi gördün? = Whom did you see?

In your sentence, it is kim, not kimi, because it is the doer, not the object.

Uydurdu is the simple past form of uydurmak.

Breakdown:

• uydur- = verbal stem, meaning make up / invent / fabricate
• -du = past tense suffix

So:

• uydurmak = to make up, to invent
• uydurdu = he/she/it made up

In Turkish, the simple past tense is often formed with:

• -dı / -di / -du / -dü
• or -tı / -ti / -tu / -tü

The form changes because of vowel harmony and consonant voicing.

Here, uydurdu means:

• he/she made up
• or, in context with kim, who made up...?

Turkish usually does not require an explicit pronoun like he/she because the verb form and context are enough.

Because uydurmak is the dictionary form (the infinitive), meaning to make up.

In a real sentence, Turkish usually uses a conjugated verb, not the infinitive.

So:

• uydurmak = to make up
• uydurdu = made up

Since the sentence is asking about a completed action in the past, Turkish uses the past-tense verb uydurdu.

No. Uydurmak is a broader verb meaning to invent, to fabricate, or to make up.

It can be used for:

• a story
• an excuse
• a lie
• a rumor
• a name
• even something improvised in certain contexts

Examples:

• Bir hikâye uydurdu. = He/she made up a story.
• Bahane uydurma! = Don’t make excuses up!
• Yalan uydurdu. = He/she fabricated a lie.

So in your sentence, kim uydurdu? has the sense of who invented/fabricated this rumor?

Because Turkish modifiers usually come before the noun they describe.

So:

• bu modifies the noun: this
• asılsız modifies the noun: unfounded
• söylenti is the noun: rumor

Together:

• bu asılsız söylenti = this unfounded rumor

This is similar to English, where adjectives also usually come before the noun:

• this unfounded rumor

One difference is that Turkish adjectives do not change for singular/plural or gender.

Yes. That is a very close translation.

Possible English versions include:

• Who made up this unfounded rumor?
• Who fabricated this baseless rumor?
• Who invented this groundless rumor?

The best choice depends on tone:

• made up = common, natural English
• fabricated = slightly more formal or forceful
• baseless/groundless/unfounded = all good translations of asılsız

So yes, fabricated this baseless rumor is very close to the Turkish wording.

Yes, Turkish allows some flexibility, but the emphasis changes.

For example:

• Bu asılsız söylentiyi kim uydurdu?
  Neutral and very natural: Who made up this unfounded rumor?

• Kim bu asılsız söylentiyi uydurdu?
  Also natural; puts kim a bit more prominently first.

Both are possible. Turkish word order is often used to manage focus and emphasis, not just grammar.

However, the version with the verb at the end is especially standard and expected.

Because Turkish does not have to copy English question-word placement.

In English, question words usually come first:

• Who made up this rumor?

In Turkish, question words often appear in the position that matches their role in the sentence, while the verb still tends to stay at the end.

So:

• Bu asılsız söylentiyi kim uydurdu?

is perfectly normal.

Turkish learners coming from English often expect kim to have to come first, but it does not.

Yes, söylenti is a standard word for rumor.

A very common synonym is dedikodu, but there is a difference in feel:

• söylenti = rumor, report, hearsay; often more neutral or formal
• dedikodu = gossip; often more informal and personal

So asılsız söylenti sounds like an unfounded rumor and fits well in serious or neutral speech.

A rough pronunciation guide is:

• Bu ≈ boo
• asılsız ≈ uh-SUHL-suhz
• söylentiyi ≈ suylen-tee-YEE
• kim ≈ kim
• uydurdu ≈ ooy-DOOR-doo

A more important point than perfect English-style spelling is stress and flow:

Bu asılsız söylentiyi kim uydurdu?

Turkish pronunciation is generally quite regular, and each written vowel is pronounced. Also, the final r in uydurdu is pronounced, though lightly in natural speech.

A simple breakdown is:

• Bu = this
• asılsız = unfounded / baseless
• söylenti-yi = rumor-ACC
• kim = who
• uydur-du = make.up-PAST

So the sentence structure is:

• [this unfounded rumor] [who] [made up]

Or more grammatically:

• Object + Subject-question word + Verb

That is why the sentence may feel unusual to an English speaker at first, even though it is completely natural Turkish.