В безударном слоге гласная часто звучит иначе, чем в ударном.

Both phrases mean in ... and both use the preposition в with the prepositional case.

• в безударном слоге = in an unstressed syllable
• в ударном = in a stressed one

In the second phrase, the noun слоге is omitted because it is understood from the first part. This is very common in Russian.

So the full version would be:

• В безударном слоге гласная часто звучит иначе, чем в ударном слоге.

But Russians usually shorten it to в ударном.

Because it agrees with слоге, which is:

• masculine
• singular
• in the prepositional case

The base adjective is безударный = unstressed.

Its prepositional singular masculine form is:

• безударный → безударном

So:

• в безударном слоге = in an unstressed syllable

The noun слог means syllable.

After в meaning in, Russian usually uses the prepositional case, so:

• слог → в слоге

That is why you see слоге, not слог.

Безударный means unstressed.

It is built from:

• без- = without
• удар / ударение = stress

So literally it means something like without stress.

In pronunciation and grammar, безударный слог is an unstressed syllable.

Yes. Ударном is short for ударном слоге.

The adjective ударный means stressed here, and the noun слог is omitted because it was already mentioned earlier.

So:

• в ударном = in the stressed one
• fully: в ударном слоге

This kind of omission is very normal in Russian.

Yes, historically it is an adjective, but in modern Russian гласная is also used as a noun meaning vowel.

You may also hear:

• гласная буква = vowel letter
• гласный звук = vowel sound

But very often Russian simply says:

• гласная = vowel

So in this sentence, гласная is the subject and means a vowel.

Because the subject is singular:

• гласная = a vowel

So the verb is also singular:

• гласная звучит = a vowel sounds

If the subject were plural, you would get:

• гласные звучат = vowels sound

Часто means often.

It is an adverb modifying the verb звучит:

• гласная часто звучит иначе = a vowel often sounds differently

So it tells you how frequently this happens.

Иначе means differently or otherwise.

The pattern иначе, чем means:

• differently from
• different than

So:

• звучит иначе, чем в ударном
  = sounds differently than in the stressed one

It is a comparison.

In this sentence, чем introduces the second part of a comparison:

• иначе, чем ... = differently than ...

Russian normally places a comma before чем in this kind of structure.

So the comma is standard punctuation here.

The stresses are:

• в безуда́рном сло́ге гла́сная ча́сто звучи́т ина́че, чем в уда́рном

Word by word:

• безуда́рном
• сло́ге
• гла́сная
• ча́сто
• звучи́т
• ина́че
• уда́рном

This is especially useful because the sentence itself is about stress.

Primarily about pronunciation.

The key verb is звучит = sounds.

So the sentence is saying that in an unstressed syllable, a vowel is often pronounced differently than in a stressed syllable.

This refers to vowel reduction, which is very important in Russian pronunciation.

Yes. A classic example is the letter о.

When stressed:

• до́м = house
• о sounds clearly like o

When unstressed, о is often pronounced more like a or a reduced vowel:

• молоко́ is pronounced roughly малако́

So the sentence is describing this general phenomenon: vowels often sound different when they are not stressed.

Yes, Russian word order is flexible.

The original sentence is neutral and natural:

• В безударном слоге гласная часто звучит иначе, чем в ударном.

But you could rearrange it for emphasis, for example:

• Гласная в безударном слоге часто звучит иначе, чем в ударном.

That version is also natural. The meaning stays basically the same.

Repeating it would be correct:

• В безударном слоге гласная часто звучит иначе, чем в ударном слоге.

This is fully grammatical.

Russian often omits repeated words when they are obvious from context, so в ударном sounds more concise and natural.

They are related but not the same part of speech.

• ударение = stress or word stress
  This is a noun.
• ударный = stressed
  This is an adjective.

So:

• ударение в слове = stress in a word
• ударный слог = a stressed syllable
• безударный слог = an unstressed syllable

In this sentence, it is most naturally understood as a vowel sound, because the verb is звучит = sounds.

So the idea is about how a vowel is pronounced in stressed vs. unstressed syllables.

In other contexts, гласная can sometimes mean a vowel letter, but here pronunciation is clearly the focus.