El requisito principal para renovar el carné es llevar una fotocopia.

Because the sentence is talking about the main requirement in a specific context, not just a main requirement among many.

• El requisito principal = the main requirement
• Un requisito principal would sound less natural here, because principal usually points to the most important one.

So the sentence presents this as the key requirement for renewing the document.

Para often means for or in order to.

Here, para renovar means to renew or in order to renew.

Spanish commonly uses an infinitive after para when the subject is general or not explicitly stated:

• para renovar el carné = to renew the licence/card

This is very normal Spanish structure:

• para estudiar = to study
• para viajar = to travel
• para trabajar = to work

In Spain, carné usually means some kind of card, licence, or permit, depending on context.

Examples:

• carné de conducir = driving licence
• carné de identidad = ID card
• carné de estudiante = student card

In your sentence, el carné probably refers to some specific official document that needs renewing. The exact English translation depends on context.

Also note the spelling:

• carné has an accent because the stress falls on the last syllable.
• In Spain, carné is the standard adapted spelling from French carnet.

Spanish can use an infinitive as the complement of ser.

So:

• El requisito principal es llevar una fotocopia
  literally means
• The main requirement is to bring a photocopy

This is very common:

• Mi sueño es viajar = My dream is to travel
• La mejor opción es esperar = The best option is to wait

In English, we also often use to + verb after is, so this structure is quite similar.

Here it means to bring.

Llevar has several related meanings, including:

• to carry
• to take
• to bring
• to wear

In administrative contexts like this, llevar una fotocopia means to bring a photocopy with you.

For example:

• Tienes que llevar el pasaporte. = You have to bring your passport.
• Lleva una foto. = Bring a photo.

So in this sentence, wear a photocopy is obviously not the meaning.

Because Spanish usually uses an article with countable nouns when talking about one item.

• una fotocopia = a photocopy

Saying just llevar fotocopia would sound incomplete in standard Spanish unless it appeared in a very reduced note-like style.

Compare:

• Necesitas una foto. = You need a photo.
• Trae una copia. = Bring a copy.

Not exactly.

• fotocopia = photocopy
• copia = copy

A fotocopia is a specific kind of copy, usually a paper copy made from a copier or scanner/printer.
A copia is more general and could refer to any copy.

So una fotocopia is more precise.

Yes, you could, although the style changes slightly.

• El requisito principal para renovar el carné...
• El requisito principal para la renovación del carné...

Both are correct.

The version with para renovar is often a bit more direct and natural in everyday language.
The version with para la renovación sounds a little more formal or bureaucratic.

This is a common contrast in Spanish:

• para solicitar la ayuda = to apply for the aid
• para la solicitud de la ayuda = for the application for the aid

The sentence is structured like this:

• El requisito principal para renovar el carné = subject
• es = linking verb
• llevar una fotocopia = complement

So it works like:

• The main requirement for renewing the card/licence is to bring a photocopy.

Spanish word order here is very natural and quite close to English.

You could rearrange it for emphasis, but this version is the most neutral and standard.

Usually renovar means to renew, not to replace.

So:

• renovar el carné = renew the licence/card

If you mean replace because it was lost, damaged, or changed, Spanish might use other expressions depending on the context, such as:

• sustituir
• reponer
• sacar un duplicado

So learners should normally understand renovar as renew.

Yes, this sounds natural, especially in official or administrative contexts.

It has a slightly formal tone because of words like:

• requisito
• renovar
• fotocopia

That is exactly the kind of language you often see or hear in offices, websites, notices, or instructions.

A less formal everyday version might be something like:

• Lo principal para renovar el carné es llevar una fotocopia.
• Para renovar el carné, lo más importante es llevar una fotocopia.

But your original sentence is perfectly normal.