Şirket asılsız iddiayı hemen yalanladı.

Because iddiayı is the definite direct object of the verb.

The base noun is iddia = claim / allegation.
In Turkish, when a direct object is specific or definite, it usually takes the accusative ending.

• iddia = a claim / claim in general
• iddiayı = the claim / that specific claim

The extra y is a buffer consonant. Turkish inserts y when a vowel-final noun takes a vowel-initial suffix:

• iddia + ı → iddiayı

So here, asılsız iddiayı means the baseless claim/allegation as a specific object.

Şirket here means the company or company depending on context.

Turkish often leaves nouns without an article, because Turkish does not have a word exactly like English the. A bare noun can be understood as definite from context.

• Şirket = company / the company
• bir şirket = a company, one company

In this sentence, Şirket is the subject, and it is naturally understood as the company from context.

Asılsız means baseless, groundless, or without foundation.

It comes from:

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

So asılsız literally means something like without basis/foundation.

That is why asılsız iddia means baseless claim or unfounded allegation.

Because Turkish adjectives normally come before the noun they describe, just like in English.

• asılsız iddia = baseless claim
• büyük şirket = big company
• hızlı araba = fast car

When the noun gets a case ending, the adjective usually stays unchanged:

• asılsız iddia
• asılsız iddiayı

So only the noun iddia takes the accusative ending here.

Yalanladı is in the simple past tense.

It breaks down like this:

• yalanla- = to deny / to call false
• -dı = past tense marker

So:

• yalanladı = he/she/it denied

Since the subject is şirket, the meaning is the company denied.

The past tense suffix appears in different forms because of vowel harmony and consonant voicing:

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

Here it is -dı.

That is a very common and very good question.

Yes, yalan by itself means lie or falsehood.
But the verb yalanlamak means to call something false, so in natural English it is usually translated as to deny or to refute.

So:

• yalan = lie / falsehood
• yalanlamak = to say something is false, to deny

In this sentence, the company is not lying; it is denying the allegation.

Hemen means immediately / right away. In Turkish, adverbs often come before the verb, especially in neutral word order.

So:

• Şirket asılsız iddiayı hemen yalanladı. = The company immediately denied the baseless allegation.

This placement is very natural in Turkish.
You may also see different word orders for emphasis, because Turkish word order is flexible.

For example:

• Şirket hemen asılsız iddiayı yalanladı.
• Asılsız iddiayı şirket hemen yalanladı.

These can all be grammatical, but the original sentence is a very normal neutral pattern.

The basic word order is:

Subject + Object + Adverb + Verb

• Şirket = subject
• asılsız iddiayı = object
• hemen = adverb
• yalanladı = verb

Turkish often places the verb at the end of the sentence. That is one of the most important patterns for English speakers to get used to.

So while English often uses:

• The company immediately denied the baseless allegation.

Turkish naturally prefers:

• The company + the baseless allegation + immediately + denied

Yes, but it would change the nuance.

Compare:

• Şirket asılsız iddiayı hemen yalanladı. = The company denied the/specific baseless claim.

• Şirket asılsız iddia yalanladı. This is not natural in standard Turkish as written.

More realistically, with an indefinite object, Turkish would use a structure like:

• Şirket asılsız bir iddiayı yalanladı. = The company denied a baseless claim.
  But because it still has -yı, it is still a specific object in the sentence.

Or:

• Şirket asılsız iddialar olduğunu yalanladı. = The company denied that there were baseless allegations.

In your sentence, iddiayı strongly suggests a particular allegation already known in context.

Because nothing in the sentence requires plurality.

• Şirket = company
• asılsız iddiayı = the baseless claim/allegation
• hemen = immediately
• yalanladı = denied

If it were plural, you might get:

• Şirket asılsız iddiaları hemen yalanladı. = The company immediately denied the baseless allegations.

Here:

• iddia → iddialar = claims/allegations
• iddiaları = the claims/allegations (as definite object)

A few sounds here are especially important for English speakers:

• ş is like sh in shoe

  • Şirket sounds roughly like shir-ket

• ı is the Turkish dotless i

  • It does not sound like English ee
  • It is a deeper, central vowel, somewhat like the vowel in the unstressed part of sofa, but not exactly

• hemen is pronounced approximately heh-men

• yalanladı

  • roughly ya-lan-la-duh
  • the final ı is again the dotless ı

Also note that Turkish spelling is very regular: words are usually pronounced much closer to how they are written than in English.

Because Turkish adjectives do not agree with nouns in the way they do in some other languages.

That means the adjective usually stays the same regardless of:

• singular/plural
• case
• gender (Turkish has no grammatical gender here)

So:

• asılsız iddia = baseless claim
• asılsız iddiayı = the baseless claim
• asılsız iddialar = baseless claims
• asılsız iddiaları = the baseless claims

The adjective asılsız stays the same every time.

Yes. Depending on context, yalanlamak can be translated as:

• deny
• refute
• reject as false
• call false

In news-style Turkish, yalanlamak is very common when a person, company, or institution says an accusation or report is not true.

So in this sentence, possible English translations include:

• The company immediately denied the baseless allegation.
• The company immediately refuted the baseless claim.
• The company promptly rejected the allegation as false.

All of those are close in meaning, though denied is probably the most straightforward.

Yes, it sounds very natural in formal written Turkish, especially in journalism or reporting.

Several features make it sound news-like:

• şirket as an institutional subject
• asılsız iddia as a formal phrase
• hemen yalanladı as a reporting expression

You will often see similar patterns in headlines and articles, such as:

• Bakan iddiaları yalanladı.
  = The minister denied the allegations.

• Şirket suçlamaları reddetti.
  = The company rejected the accusations.

So this is a very useful sentence pattern to learn.