El cerdo gruñe cuando oye demasiado ruido.

Because Spanish often uses the definite article to talk about a whole class of animals or things in a general way.

So el cerdo can mean the pig in a general sense, like pigs as a type of animal.

Compare:

• El cerdo gruñe... = The pig grunts... / A pig grunts... / Pigs grunt... in a general statement
• Los cerdos gruñen... = Pigs grunt...

Spanish likes this generic use of el / la / los / las more than English does.

Gruñir is the infinitive, meaning to grunt or to growl.

In the sentence, the verb has to be conjugated because the subject is el cerdo.
For él / ella / usted, the present tense form is gruñe.

So:

• gruñir = to grunt
• el cerdo gruñe = the pig grunts

This is the same pattern as:

• vivir → vive
• escribir → escribe
• gruñir → gruñe

Gruñe is pronounced roughly like GROO-nyeh.

A few key points:

• gr sounds like the beginning of grow
• u is pronounced
• ñ sounds like the ny in canyon
• final e is clearly pronounced, not silent

So the word has two syllables: gru-ñe.

The letter ñ is a separate letter in Spanish, and it changes the sound completely.

• n = normal n
• ñ = a ny sound

So:

• gruñe sounds like groo-nyeh
• not groon-eh

This sound appears in many Spanish words, such as:

• niño
• España
• mañana

Because oír and escuchar are related but not identical.

• oír = to hear
  This is usually more passive: sound reaches you.
• escuchar = to listen
  This is more active: you pay attention to a sound.

In cuando oye demasiado ruido, the idea is that the pig hears too much noise, not necessarily that it is intentionally listening.

So oye is the most natural choice here.

Spanish usually does not need subject pronouns like he, she, or it unless you want emphasis or contrast.

Since oye already tells you it is he/she/it form, and the subject el cerdo is already present, adding él would normally be unnecessary.

So:

• El cerdo oye... = normal
• Él oye... = only if you want emphasis, contrast, or to avoid confusion

Spanish is a pro-drop language, which means subject pronouns are often left out.

Because oír is an irregular verb in the present tense.

Its present forms are:

• oigo
• oyes
• oye
• oímos
• oís
• oyen

You can see that most forms use oy-.

So oye is the él / ella / usted form of oír.

Because the sentence describes a habitual or general action: whenever the pig hears too much noise, it grunts.

In Spanish, when cuando refers to something that happens regularly or generally, you use the present indicative:

• El cerdo gruñe cuando oye demasiado ruido.

But if you are talking about a future event, Spanish often uses the present subjunctive after cuando:

• El cerdo gruñirá cuando oiga demasiado ruido.
  = The pig will grunt when it hears too much noise.

So here, oye is used because this is a general truth or repeated situation.

Because ruido is a masculine noun, so demasiado has to agree with it.

• ruido = masculine singular
• therefore: demasiado ruido

If the noun were feminine, you would use demasiada:

• demasiada música
• demasiada luz

Agreement is very important in Spanish.

Here demasiado means too much.

So demasiado ruido = too much noise.

It suggests an excessive amount, not just a large amount.

Compare:

• mucho ruido = a lot of noise
• demasiado ruido = too much noise

So demasiado adds the idea that the noise is more than is comfortable or acceptable.

Because ruido is often treated as an uncountable noun, like noise in English.

So:

• demasiado ruido = too much noise

You would not usually make it plural unless you meant different individual noises or sounds in a specific context.

For a general idea of noise, singular is the normal choice.

Yes, cerdo can refer to:

• the animal: pig
• pork, in some contexts

But in this sentence, because el cerdo is the subject and performs the action gruñe, it clearly means the pig as an animal.

Food does not grunt, so the context removes any ambiguity.

Yes. Gruñir can mean to grunt, to growl, or more generally to make a grunting/growling sound, depending on the animal or context.

For a pig, grunt is usually the best English translation.
For a dog, growl might be more natural.

So the exact English word can change, even though the Spanish verb stays the same.

Yes, that is possible, but it changes the nuance slightly.

• cuando oye demasiado ruido = when it hears too much noise
• si oye demasiado ruido = if it hears too much noise

In many contexts they can be close, but:

• cuando sounds more like a repeated or expected situation
• si sounds more conditional

In this sentence, cuando is very natural because it describes what happens in that circumstance as a regular pattern.