La profesora dice que un adverbio bien elegido puede sonar más natural que una frase muy larga.

In Spanish, you usually use the definite article with a specific person when you identify them by their role or job: la profesora = the teacher.

Also, profesora is the feminine form of profesor. If the teacher were male, it would be el profesor.

A useful contrast:

• La profesora dice... = The teacher says...
• Es profesora. = She is a teacher.

After ser, the article is often omitted.

After verbs like decir, Spanish normally uses que to introduce what someone says or thinks.

So:

• La profesora dice que... = The teacher says that...

In English, you can often omit that:

• The teacher says a well-chosen adverb...

In Spanish, you normally keep que.

This second que means than.

So the sentence contains two different ques with two different jobs:

• dice que = says that
• más natural que = more natural than

This is very common in Spanish.

Because adverbio is a masculine noun, so it takes un in the singular:

• un adverbio
• el adverbio

It also means an adverb in the grammar sense.

The sentence uses un because it is talking about any well-chosen adverb, not one specific adverb already known to the listener.

Bien elegido means well chosen.

Here is how it works:

• elegido is the past participle of elegir = to choose
• in this sentence, elegido is being used like an adjective
• bien is an adverb modifying elegido

So:

• un adverbio elegido = a chosen adverb
• un adverbio bien elegido = a well-chosen adverb

It is bien, not bueno, because bien modifies elegido.
Bueno is an adjective, not an adverb.

Also, elegido ends in -o because it agrees with adverbio, which is masculine singular.

Because after poder, Spanish uses an infinitive.

The pattern is:

• poder + infinitive

So:

• puede sonar = can sound / may sound
• not puede suena

More examples:

• puede hablar = can speak
• puede parecer = can seem
• puede mejorar = can improve

Because this sentence is making a normal statement, so Spanish uses the indicative, not the subjunctive.

• La profesora dice que... puede sonar...

After decir que, if you are reporting information as a fact or opinion, the indicative is normal.

The subjunctive would appear in different kinds of structures, for example with doubt, wishes, or certain negative expressions:

• No creo que pueda sonar natural.
• Quiere que suene más natural.

So here, puede is the expected form.

Yes, but in this context it is also used in the broader sense of to sound natural, to come across, or to seem to the ear.

In language-learning Spanish, sonar is very common:

• Suena bien = It sounds good
• Suena raro = It sounds strange
• Suena natural = It sounds natural

So puede sonar más natural means something like can sound more natural or can come across as more natural.

Because natural is describing how the whole expression sounds, so Spanish uses an adjective after sonar.

• sonar natural = to sound natural

If you used naturalmente, that would be an adverb meaning naturally, and it would create a different structure and meaning.

Also, natural has the same form for masculine and feminine singular:

• un adverbio natural
• una frase natural

Only the plural changes:

• naturales

In this sentence, it refers to un adverbio, so masculine singular is appropriate.

Because muy is used before adjectives and adverbs:

• muy larga
• muy natural
• muy bien

Mucho is used differently:

• with nouns: mucha paciencia
• with verbs: habla mucho

So here, since larga is an adjective, muy is correct.

Also, larga agrees with frase, which is feminine singular:

• una frase larga
• unas frases largas

Because in Spanish, descriptive adjectives usually come after the noun.

So these are the neutral, standard orders:

• un adverbio bien elegido
• una frase muy larga

Spanish can sometimes place adjectives before the noun, but that often changes the tone or gives a more literary or subjective feel.

For example:

• una frase muy larga = the normal, straightforward order
• una larga frase = possible, but more stylistic

So the word order in your sentence is the most natural everyday choice.