En la segunda estrofa, la rima cambia y el poema se vuelve más triste.

En la segunda estrofa means in the second stanza.

Spanish often uses en to mean in when talking about parts of a text, song, poem, chapter, etc. So:

• en la primera estrofa = in the first stanza
• en el segundo capítulo = in the second chapter
• en la última línea = in the last line

It sets the scene for where this change happens in the poem.

In Spanish, ordinal numbers like primero, segundo, tercero usually come before the noun:

• la primera estrofa
• el segundo verso
• la tercera línea

So la segunda estrofa is the normal word order.

Putting the ordinal after the noun is unusual here and would sound literary, old-fashioned, or marked in some contexts.

Spanish uses definite articles much more often than English.

Here, la segunda estrofa refers to a specific, identifiable stanza: the second stanza. So the article is required.

Compare:

• la segunda estrofa = the second stanza
• un poema = a poem
• el poema = the poem

In this sentence, both la segunda estrofa and el poema are specific.

Estrofa means stanza.

In poetry vocabulary:

• estrofa = stanza
• verso = line / verse
• rima = rhyme
• poema = poem

So the sentence is talking about a structural part of a poem.

Cambia is the present indicative, third person singular, from cambiar.

It matches la rima, which is singular:

• la rima cambia = the rhyme changes

Other forms would mean different things:

• cambie = present subjunctive
• cambió = preterite, changed

The present tense is often used in literary commentary, even in English too:
In the second stanza, the rhyme changes...

Both are possible, but la rima cambia is the more neutral order.

Spanish usually prefers subject + verb in straightforward statements:

• La rima cambia
• El poema se vuelve más triste

You could also say cambia la rima, but that puts slightly more focus on the action or sounds a bit more stylistically marked.

Here, se vuelve means becomes.

The verb is volverse, which is a very common way to express a change of state:

• se vuelve triste = becomes sad
• se vuelve difícil = becomes difficult
• se vuelve oscuro = becomes dark

So el poema se vuelve más triste means the poem becomes sadder.

Because the verb here is volverse, not just volver.

These are different:

• volver = to return / to turn
• volverse = to become

Examples:

• Vuelve a casa = He/She returns home
• El cielo se vuelve gris = The sky becomes grey

So the se is part of the verb’s meaning here. It is not optional.

Yes, you could, and it would still mean something like becomes sadder, but the nuance can be slightly different.

• ponerse often suggests a change of state, sometimes more immediate or temporary
• volverse can suggest a development or transformation

In literary commentary, se vuelve más triste sounds very natural and elegant.

So:

• el poema se pone más triste = the poem gets sadder
• el poema se vuelve más triste = the poem becomes sadder

Both are understandable, but se vuelve fits very well in analysis of literature.

Because triste is an adjective with the same form for masculine and feminine singular.

So you get:

• el poema es triste
• la canción es triste

No change is needed.

Also, after más, the adjective stays in its normal form:

• más triste = sadder / more sad
• más feliz = happier
• más oscuro = darker

So más triste is correct.

Spanish usually forms comparatives with más + adjective rather than with an -er ending like English.

So:

• más triste = sadder
• más largo = longer
• más intenso = more intense

This is one of the most common comparison patterns in Spanish.

Because poema is a masculine noun, even though it ends in -a.

Some nouns of Greek origin ending in -ma are masculine in Spanish:

• el poema
• el problema
• el sistema
• el tema

So it must be:

• el poema
• un poema
• este poema

not la poema.

The comma separates the introductory phrase En la segunda estrofa from the main statement.

It helps readability:

• En la segunda estrofa, la rima cambia...

This kind of comma is common when a sentence starts with a time/place/context phrase. It is not always absolutely required in every short sentence, but here it is very natural and clear.

Because Spanish often avoids repeating the subject when it is already clear.

Here the sentence has two different subjects:

1. la rima cambia
2. el poema se vuelve más triste

In the second part, the subject el poema is stated because it is different from la rima.

If both parts had the same subject, Spanish would often omit the second one:

• El poema cambia y se vuelve más triste

But in your sentence, the subjects are different, so both are mentioned.

Yes. Y means and, and it can connect words, phrases, or complete clauses.

Here it links two coordinated statements:

• la rima cambia
• el poema se vuelve más triste

So the structure is perfectly normal: A changes and B becomes sadder.

Yes, very much so.

Spanish, like English, often uses the present tense when discussing literature, films, and other works:

• En esta escena, el protagonista descubre la verdad.
• Al final del poema, el tono cambia.
• La rima cambia y el poema se vuelve más triste.

This is sometimes called the literary present. It makes the analysis sound immediate and standard.