La profesora explicó la metáfora con calma, y de pronto entendí por qué ese verso sonaba tan triste.

This is one of the most important grammar points in the sentence.

• explicó and entendí are in the preterite because they present completed events:

  • La profesora explicó... = the teacher explained it
  • entendí = I understood it / I got it

• sonaba is in the imperfect because it describes an ongoing quality or background impression:

  • ese verso sonaba tan triste = that line sounded so sad

So the sentence combines:

• main completed actions: the explanation happened, and then understanding happened
• background description: the verse had a sad sound/tone

That mix is very natural in Spanish.

Because the sentence is not talking about one single completed sound the verse made. It is talking about how the verse came across or seemed.

• sonaba triste = it sounded sad, it had a sad tone
• sonó triste = it sounded sad at one specific moment, as a finished event

With things like descriptions, impressions, and atmosphere, Spanish often prefers the imperfect:

• parecía raro
• se veía cansado
• sonaba triste

So sonaba fits the idea of an ongoing impression.

Because Spanish often leaves out subject pronouns when the verb ending already makes the subject clear.

• entendí already tells you it means I understood
• the ending -í marks first person singular in the preterite

So entendí is enough by itself.

You could say yo entendí, but that would usually add emphasis, contrast, or clarification:

• Yo entendí, pero ellos no.

In a normal sentence like this, leaving out yo sounds natural.

Because this is an indirect question: I understood why...

• por qué = why
• porque = because

So:

• No sé por qué vino. = I don’t know why he came.
• Vino porque quería hablar. = He came because he wanted to talk.

In your sentence:

• entendí por qué ese verso sonaba tan triste
• literally: I understood why that line sounded so sad

The accent on qué is important here because it marks an interrogative word, even inside an indirect question.

Con calma literally means with calm, but in natural English it means something like:

• calmly
• unhurriedly
• patiently

Spanish often uses phrases like con + noun where English might prefer an adverb.

So:

• explicó la metáfora con calma = she explained the metaphor calmly / patiently

You could also say calmadamente, but con calma is more common and more natural in everyday Spanish.

Yes, in this sentence de pronto means:

• suddenly
• all of a sudden

So:

• y de pronto entendí... = and suddenly I understood...

It is very close in meaning to de repente. Both are common. In many contexts they are interchangeable.

Examples:

• De pronto, se hizo el silencio.
• De repente, se hizo el silencio.

Spanish demonstratives show distance, but not only physical distance. They can also show mental or contextual distance.

• este = this, closer to the speaker
• ese = that, a bit farther away or already mentioned
• aquel = that over there / more distant

Here, ese verso probably refers to a line being discussed in class or in the text. Ese works well for something already identifiable in the conversation.

So ese verso feels like:

• that line we’re talking about
• that particular verse

Verso usually means a line of poetry or a verse.

In literary contexts:

• poema = poem
• verso = line/verse within a poem

So in this sentence, ese verso most likely refers to one specific poetic line the teacher was explaining.

It is not the same as versículo, which is more often used for numbered verses in texts like the Bible.

Both are possible, but they are not exactly the same.

• muy triste = very sad
• tan triste = so sad

Tan often sounds more expressive and more tied to the speaker’s reaction or discovery.

So:

• sonaba muy triste = it sounded very sad
• sonaba tan triste = it sounded so sad

In your sentence, tan triste fits well because the speaker has a moment of realization:

• de pronto entendí por qué...
• suddenly I understood why it sounded so sad

Because Spanish uses definite articles very naturally with specific nouns.

Here:

• la profesora = the teacher
• la metáfora = the metaphor

This suggests that both are specific and identifiable in context:

• the teacher in that situation
• the metaphor they were discussing

Spanish often uses articles where English might sometimes omit them, but in this sentence the article is completely expected.

Also, profesora is the feminine form of profesor, so it tells us the teacher is female.

They are there for standard Spanish spelling rules, but each one has a slightly different reason.

• metáfora has an accent because it is stressed on the third-to-last syllable: me-TÁ-fo-ra. Words stressed there always need a written accent.
• entendí has an accent to show the stress on the final -í: en-ten-DÍ.
• qué has an accent because it is an interrogative word here, in an indirect question: por qué = why.

So the accents are not optional; they are part of correct spelling.

Not always, but here it is acceptable.

Spanish usually does not put a comma before y in simple lists or short linked clauses. However, a comma can appear before y when:

• the clauses are a bit longer
• there is a natural pause
• the subject changes
• the writer wants to separate ideas more clearly

In this sentence:

• first clause: La profesora explicó la metáfora con calma
• second clause: de pronto entendí por qué ese verso sonaba tan triste

The subject changes from la profesora to I (understood from entendí), so the comma is reasonable. Many writers would also write it without the comma, but with the comma it still looks natural.