Es una buena oportunidad para aprender algo nuevo.

Because Spanish normally uses ser for general definitions, evaluations, and identifying what something is.

Here, Es una buena oportunidad means It is a good opportunity as a general judgment.

Using está would sound wrong in this sentence, because estar is usually for location, temporary states, or conditions. So:

• Es una buena oportunidad = It is a good opportunity
• not Está una buena oportunidad

Spanish often leaves out the subject when it is obvious or unimportant.

So Es una buena oportunidad... literally has no spoken subject, but in English we usually need one, so we translate it as It is a good opportunity...

Depending on context, the hidden idea could be:

• Esto es... = This is...
• Eso es... = That is...
• or just an impersonal it is

Leaving the subject out is completely normal in Spanish.

Because oportunidad is a feminine singular noun, so the article must match it:

• una oportunidad
• not un oportunidad

Spanish articles agree with the noun in gender and number:

• un libro
• una casa
• unos libros
• unas casas

Because oportunidad is feminine.

The adjective bueno changes to match the noun:

• masculine singular: bueno
• feminine singular: buena
• masculine plural: buenos
• feminine plural: buenas

So:

• una buena oportunidad = correct
• un buen libro = correct

A useful detail: bueno shortens to buen only before a masculine singular noun:

• un buen día
• but una buena idea

Yes, adjectives often come after the noun in Spanish, but some very common adjectives can also go before it.

With bueno / buena, putting it before the noun is very natural when giving a general positive evaluation:

• una buena oportunidad = a good opportunity

If you said una oportunidad buena, it would sound less natural here and more contrastive, as if you were distinguishing it from other opportunities.

So in this sentence, una buena oportunidad is the normal phrasing.

Para + infinitive often expresses purpose or intended use.

So:

• para aprender = to learn / for learning / in order to learn

In this sentence, it tells us what kind of opportunity it is:

• Es una buena oportunidad para aprender algo nuevo.
• It is a good opportunity to learn something new.

This pattern is very common:

• Tengo tiempo para estudiar. = I have time to study.
• Necesito dinero para viajar. = I need money to travel.

Because in Spanish, adjectives usually come after words like algo, nada, mucho, and similar expressions.

So the normal order is:

• algo nuevo = something new
• algo interesante = something interesting
• nada bueno = nothing good

Nuevo algo is not correct here.

Even though algo means something, it does not behave like a feminine noun.

Words like algo, nada, esto, eso, and aquello usually take the adjective in the masculine singular form:

• algo nuevo
• nada raro
• esto es bueno

So nuevo is the normal form after algo.

Yes, grammatically you could, but algo nuevo is more natural and more common in this kind of sentence.

Compare:

• aprender algo nuevo = learn something new
• aprender una cosa nueva = learn a new thing

The second version is understandable, but it sounds a bit more literal and less idiomatic in many contexts. Spanish usually prefers algo nuevo here.