Lo de corear el estribillo durante el dúo fue su idea.

Here lo de is a very common Spanish structure meaning something like the thing about..., that business of..., or the matter of....

So Lo de corear el estribillo durante el dúo is not just lo = it.
The whole chunk lo de + infinitive/noun turns an idea or situation into a topic.

Common patterns:

• Lo de llegar tarde = the thing about arriving late
• Lo de Marta = the thing with Marta / what happened with Marta
• Lo de corear el estribillo... = the thing about chanting the chorus...

It often sounds slightly informal or conversational.

Because Spanish often uses the infinitive as a noun-like form.

Here, corear el estribillo durante el dúo functions as a whole idea or action, not as the main verb of the sentence.

Compare:

• Fumar es malo. = Smoking is bad.
• Cantar me relaja. = Singing relaxes me.
• Lo de corear el estribillo... = the thing about chanting the chorus...

So corear here is not saying who did the action directly; it is naming the action itself.

Because Spanish uses articles more often than English, especially when referring to a specific thing.

Here el estribillo means the chorus/refrain, most likely a specific one that both speaker and listener know about.

So even if English might sometimes sound natural with chanting chorus lines or something similar, Spanish strongly prefers corear el estribillo when it means a particular chorus.

Also, corear takes a direct object, and that object here is definite:

• corear el estribillo
• corear una consigna
• corear su nombre

Corear usually means to chant, cheer, or sing along with something in a repeated or group way.

In music-related contexts, it can mean joining in on a repeated line, especially the chorus or refrain. In crowds, it can also mean chanting slogans or names.

Examples:

• El público coreó la canción.
• Los fans coreaban su nombre.
• Todos corearon el estribillo.

So it suggests a collective or repeated vocal action, not just quietly singing.

Usually, yes. Estribillo is the standard word for chorus or refrain in a song.

Depending on context, it can refer to:

• the repeated part of a song
• a refrain that comes back several times

In many cases, chorus is the best translation.
It is a very common music term in Spain.

Durante emphasizes during the course of the duet, focusing on time.

So:

• durante el dúo = during the duet
• en el dúo = in the duet

Both can be possible in some contexts, but they do not feel exactly the same.

Here, durante highlights when the action happened:

• the chanting happened while the duet was going on

If you said en el dúo, it would sound a bit more like locating the action within that musical section rather than emphasizing its duration.

Fue is the preterite of ser, and it presents this as a completed fact: at that moment, it was his/her/their idea.

Spanish often uses:

• fue for a specific event or conclusion
• era for background description, habitual situations, or an ongoing state in the past

Here, fue su idea sounds like:

• this was the idea that person came up with
• that action originated with them

If you said era su idea, it could sound more descriptive or like background information, depending on context.
With no extra context, fue su idea is the natural choice for identifying who came up with it.

Su can mean several things:

• his
• her
• its
• your (formal singular)
• their

Spanish does not distinguish these in the possessive adjective su. The context tells you which one is meant.

If the speaker wants to remove ambiguity, they can say:

• idea de él
• idea de ella
• idea de ellos
• idea de usted

So fue su idea is grammatically clear, but not always specific without context.

The subject is the whole phrase:

Lo de corear el estribillo durante el dúo

That entire idea is what fue su idea refers to.

So the structure is:

• Subject: Lo de corear el estribillo durante el dúo
• Verb: fue
• Predicate/complement: su idea

This is why the sentence can feel long at the front: Spanish is treating the whole action/topic as one unit.

Yes, you could, and it is very natural.

There is a slight difference in feel:

• fue su idea = straightforward, neutral
• fue idea suya = often a bit more emphatic, as in it was their idea

Both are correct.

Compare:

• Lo de corear el estribillo durante el dúo fue su idea.
• Lo de corear el estribillo durante el dúo fue idea suya.

The second version can put a little more focus on who was responsible for the idea.

Because dúo is pronounced with two separate vowel sounds: dú-o.

The accent mark shows the stress and helps indicate that the u and o do not merge into a single syllable. This creates a hiatus.

So it has two syllables:

• dú-o

Without the accent, the pronunciation rules would suggest something different. The written accent helps preserve the correct stress and pronunciation.