Esta balada es muy larga.

Because balada is a feminine singular noun in Spanish.

• este = this (for masculine singular nouns)
• esta = this (for feminine singular nouns)

So:

• este libro = this book
• esta balada = this ballad

Spanish demonstratives have to agree with the noun they describe.

In this sentence, you can tell because it uses esta and larga, both of which are feminine forms.

Also, many nouns ending in -a are feminine, and balada is one of them:

• la balada
• una balada
• esta balada

As always, it is best to learn nouns together with their gender.

Spanish uses ser and estar in different ways, and here ser is the natural choice.

• es comes from ser
• está comes from estar

In Esta balada es muy larga, the sentence is describing a characteristic of the ballad: its length. That is treated as a general quality, so Spanish uses ser.

So:

• Es muy larga = It is very long

Using está here would usually sound wrong in standard usage.

Because adjectives in Spanish usually agree with the noun they describe.

Since balada is feminine singular, the adjective must also be feminine singular:

• largo = masculine singular
• larga = feminine singular
• largos = masculine plural
• largas = feminine plural

Examples:

• un libro largo
• una balada larga

In your sentence, muy larga matches esta balada.

Muy means very.

It goes before adjectives and adverbs:

• muy larga = very long
• muy rápido = very fast

Unlike some other words in Spanish, muy does not change for gender or number. It always stays muy.

Because esta already works as a determiner, like this in English.

In English, you say:

• this ballad

not

• the this ballad

Spanish works the same way:

• esta balada

not

• la esta balada

So esta replaces the article.

Yes, but this version is the most straightforward and natural.

The normal structure here is:

demonstrative + noun + verb + adverb + adjective

So:

• Esta balada es muy larga

You could also say things like:

• Es muy larga, esta balada.

But that sounds more marked or expressive, not like the basic neutral version a learner should start with.

Yes, if the context already makes it clear what esta refers to.

For example, if you are pointing to a song or discussing several ballads, you could say:

• Esta es muy larga. = This one is very long.

But if the listener does not know what esta refers to, it is better to keep the noun:

• Esta balada es muy larga.

You would change the demonstrative, the verb, and the adjective so they all agree in the plural:

• Estas baladas son muy largas.

Breakdown:

• esta → estas
• balada → baladas
• es → son
• larga → largas

Muy stays the same.

A simple approximate pronunciation is:

EH-sta ba-LA-da es mwee LAR-ga

A few useful notes:

• esta → stress on ES
• balada → stress on LA
• muy sounds roughly like mwee
• larga → the g is hard, like in go

In standard Peninsular Spanish, the full sentence sounds smooth and connected, especially between es muy.