Dormir temprano es vital para aprender.

Because dormir is an infinitive here, and in Spanish an infinitive can act like the subject of a sentence.

So Dormir temprano es vital para aprender literally works like:

• Sleeping early is vital for learning
• or To sleep early is vital for learning

You do not use duermo because duermo means I sleep, and that would change the sentence completely.

Yes, in a way.

In Spanish, infinitives such as dormir, comer, estudiar can function like noun-like ideas:

• Fumar es malo. = Smoking is bad.
• Leer ayuda mucho. = Reading helps a lot.
• Dormir temprano es vital... = Sleeping early is vital...

So dormir is still a verb in form, but it is being used as the subject of the sentence.

Because temprano is being used as an adverb here, not an adjective.

It tells us when or how early the action happens:

• dormir temprano = to sleep early

As an adverb, temprano does not change form.

Compare:

• una hora temprana = an early hour
  Here temprana is an adjective and agrees with hora.
• me acuesto temprano = I go to bed early
  Here temprano is an adverb.

Because ser is used for general truths, definitions, and essential qualities.

Here the sentence expresses a general statement:

• Dormir temprano es vital para aprender.

It means that, in general, sleeping early is vital for learning.

Estar would sound wrong here because vital is not being presented as a temporary state or location.

Para + infinitive is a very common Spanish structure used to express purpose or goal.

So:

• para aprender = in order to learn / for learning

The sentence is saying that sleeping early is vital for the purpose of learning.

Other examples:

• Estudio para mejorar. = I study to improve.
• Hago ejercicio para estar bien. = I exercise to feel well.

Normally, no.

In this sentence, para is the correct choice because it expresses purpose:

• para aprender = in order to learn

Por usually has other meanings, such as:

• because of
• through
• for the sake of
• around/by

So por aprender would not fit the intended meaning here.

Usually, no.

The most natural version is simply:

• Dormir temprano es vital para aprender.

Spanish often uses a bare infinitive as the subject, without an article.

You may sometimes see el + infinitive in more formal or literary language, but it is less common in everyday speech:

• El dormir bien es importante.

That is possible, but Dormir bien es importante sounds more normal.

Not exactly.

• dormir temprano = to sleep early
• acostarse temprano = to go to bed early

In real life, Spanish speakers often say acostarse temprano when they mean someone goes to bed early, because that is usually the more precise action.

So these are both natural, but they focus on slightly different things:

• Dormir temprano es vital para aprender.
• Acostarse temprano es vital para aprender.

The second one emphasizes the act of going to bed early.

Yes, it is natural and correct.

A speaker from Spain would understand it immediately. Depending on context, they might also say:

• Dormir pronto es vital para aprender.
• Acostarse temprano es fundamental para aprender.
• Dormir bien es vital para aprender.

But your original sentence is perfectly good Spanish.

Both are possible, but vital is stronger.

• importante = important
• vital = vital / essential / crucial

So:

• Dormir temprano es importante para aprender.
  means it is important.
• Dormir temprano es vital para aprender.
  means it is extremely important or essential.

Yes, but it changes the style a little.

• para aprender = more verbal, more direct, very natural
• para el aprendizaje = more formal, more abstract

So:

• Dormir temprano es vital para aprender.
  sounds very natural in everyday Spanish.
• Dormir temprano es vital para el aprendizaje.
  sounds a bit more formal or academic.

In standard Spain Spanish, roughly:

dor-MEER tem-PRA-no es bee-TAL PA-ra a-pren-DER

A few helpful notes:

• dormir stresses the last syllable: dor-MIR
• temprano stresses pra: tem-PRA-no
• vital stresses the last syllable: vi-TAL
• aprender stresses the last syllable: a-pren-DER

None of these words needs a written accent mark because they already follow normal Spanish stress rules.