La yegua se asusta si el cerdo vuelve a gruñir cerca del corral.

Because asustarse is the common way to say to get scared / to become frightened.

• La yegua se asusta = The mare gets scared
• Asusta without se is usually transitive: it scares someone / something

Compare:

• La yegua se asusta. = The mare gets scared.
• El cerdo asusta a la yegua. = The pig scares the mare.

So in this sentence, the mare is the one experiencing the fear, which is why se is used.

Yegua means mare, that is, an adult female horse.

Some related words:

• caballo = horse / male horse
• yegua = mare
• potro = colt / young horse
• potranca = filly

So la yegua is more specific than just the horse.

In Spanish, definite articles like el and la are used more often than the is in English.

Here they help refer to specific animals in the situation:

• la yegua = the mare
• el cerdo = the pig
• el corral = the pen / corral

Even if English might sometimes sound natural without repeating the, Spanish often keeps it.

Here si means if.

This is a normal real or possible condition, so Spanish uses the indicative, not the subjunctive:

• La yegua se asusta si el cerdo vuelve a gruñir...
• The mare gets scared if the pig growls again...

Spanish generally does not use the subjunctive after si for ordinary if-clauses like this.

Examples:

• Si llueve, no salgo. = If it rains, I’m not going out.
• Si viene, hablamos. = If he comes, we’ll talk.

Because the sentence expresses a general condition or something that happens whenever that situation occurs.

• se asusta = gets scared
• vuelve = comes back / again
• gruñir = to grunt / growl

So the idea is:

The mare gets scared if the pig growls again near the corral.

This is like English sentences such as:

• She gets nervous if the dog barks.
• Ice melts if it gets warm enough.

Spanish often uses the present tense this way for habitual or repeated situations.

Volver a + infinitive means to do something again.

So:

• vuelve a gruñir = growls again / starts growling again

This is a very common structure:

• volver a intentar = to try again
• volver a hablar = to speak again
• volver a empezar = to start again

You could also say gruñe otra vez, but vuelve a gruñir is a very natural and common way to express the same idea.

Because after volver a, Spanish uses an infinitive.

Pattern:

• volver a + infinitive

So:

• vuelve a gruñir
• not vuelve a gruñe

The conjugated verb is vuelve; the second verb stays unchanged as an infinitive.

Gruñir means to grunt, to growl, or sometimes to snarl, depending on the animal and context.

For a pig, gruñir often suggests grunting.
For a dog or another aggressive animal, it may sound more like growling.

It is not the same as bark:

• ladrar = to bark
• gruñir = to growl / grunt

So for a pig, gruñir is the natural choice.

Because cerca is normally followed by de.

• cerca de = near
• lejos de = far from

So:

• cerca del corral = near the corral

Also, del is just the contraction of de + el:

• de el corral → del corral

This contraction is required in standard Spanish.

Corral means an animal pen, enclosure, or corral.

It refers to a fenced area where animals are kept. Depending on context, English might translate it as:

• corral
• pen
• enclosure

So cerca del corral means near the animal pen / near the corral.

Yes. Spanish allows some flexibility in word order, though the original sentence is very natural.

Possible variations:

• Si el cerdo vuelve a gruñir cerca del corral, la yegua se asusta.
• La yegua se asusta si el cerdo gruñe otra vez cerca del corral.

These keep roughly the same meaning.

The original version sounds straightforward and neutral:

• main idea first: La yegua se asusta
• condition after: si el cerdo vuelve a gruñir...

Here it is more like gets scared.

• asustarse often focuses on the change of state: becoming frightened
• tener miedo focuses more on the feeling of fear itself

So:

• La yegua se asusta = The mare gets scared
• La yegua tiene miedo = The mare is afraid

In this sentence, se asusta fits well because the pig’s noise causes a reaction.

You could, but it would change the meaning.

• La yegua se asusta = The mare gets scared / becomes frightened
• La yegua está asustada = The mare is scared / frightened

The first describes the reaction happening when the pig growls again.
The second describes the mare’s state.

So in this sentence, se asusta is the better choice because it expresses what happens in response to the pig’s action.