El ejemplo entre paréntesis no es obligatorio, pero ayuda a entender la definición.

Because obligatorio describes an inherent quality or status of the example, not a temporary condition.

In Spanish, ser is used for things like classification, definition, and general characteristics:

• Es obligatorio = It is mandatory / required
• Es opcional = It is optional

Using estar obligatorio would sound wrong in standard Spanish.

Because obligatorio agrees with el ejemplo, which is a masculine singular noun.

• el ejemplo → masculine singular
• so the adjective must also be masculine singular: obligatorio

Compare:

• El ejemplo es obligatorio
• La explicación es obligatoria

Even though ejemplo ends in -o, here it really is masculine, so the agreement is straightforward.

It means in parentheses or literally between parentheses.

This is a very common Spanish expression:

• entre paréntesis = in parentheses
• poner algo entre paréntesis = to put something in parentheses

It is the normal way to express this idea in Spanish, even though English usually says in parentheses rather than between parentheses.

Spanish idiomatically says entre paréntesis, not en paréntesis.

The idea is that the word or phrase is placed between the opening and closing parentheses. So even if English says in parentheses, Spanish prefers entre.

So:

• entre paréntesis = the standard expression
• en paréntesis would not be the usual choice here

Because entre paréntesis is a fixed expression.

Spanish often drops the article in established adverbial phrases like this. So you simply learn it as a chunk:

• entre paréntesis

Not:

• entre los paréntesis in this context

If you were talking about actual physical parentheses in a more literal way, the article might appear in other sentence structures, but here the fixed phrase is the natural one.

In form, paréntesis is one of those nouns that looks the same in singular and plural.

• el paréntesis = the parenthesis
• los paréntesis = the parentheses

In the expression entre paréntesis, you do not need to worry too much about whether it is singular or plural, because the whole phrase functions idiomatically as in parentheses.

Because ayudar is commonly followed by a + infinitive when it means to help to do something.

So:

• ayuda a entender = it helps (someone) to understand
• me ayuda a recordar = it helps me remember
• esto ayuda a aprender = this helps one learn

In English, to is sometimes optional after help (helps understand / helps to understand), but in Spanish the a is the normal connector here.

Because Spanish does not need to state the person explicitly here.

Ayuda a entender la definición means:

• It helps to understand the definition
• or It helps you/us/one understand the definition, depending on context

This is a general statement. Spanish often leaves the person unspecified when it is obvious or unimportant.

If you wanted to specify it, you could say:

• ayuda al lector a entender la definición = it helps the reader understand the definition

Spanish uses the definite article much more often than English does.

Here, la definición refers to a specific definition already understood from the context, so the article sounds natural:

• ayuda a entender la definición

English might say understand the definition, so this matches fairly well. But even in places where English drops the article, Spanish often keeps it.

No simply negates the verb:

• es obligatorio = it is mandatory
• no es obligatorio = it is not mandatory

In Spanish, no normally goes directly before the conjugated verb:

• no es
• no tiene
• no ayuda

That word order is very regular and important.

Because pero introduces a contrast, and in Spanish it is normally preceded by a comma when it links two clauses.

Here the sentence contrasts two ideas:

• no es obligatorio
• pero ayuda a entender la definición

So the comma is standard punctuation:

• ..., pero ...

It works much like English:

• It isn’t required, but it helps...

Yes, but the meaning changes slightly.

• obligatorio = mandatory / required
• necesario = necessary

So:

• no es obligatorio = it is not required
• no es necesario = it is not necessary

Those are close, but not identical. Something can be not mandatory and still be helpful, which is exactly the idea in this sentence.

Because Spanish often omits repeated subjects when they are clear from context.

In:

• El ejemplo entre paréntesis no es obligatorio, pero ayuda a entender la definición

the subject of both verbs is El ejemplo entre paréntesis:

• no es
• ayuda

Spanish does not need to repeat el ejemplo before ayuda, because it is already understood.

Yes. Ejemplo is a masculine noun:

• el ejemplo
• un ejemplo
• buen ejemplo

This can surprise English speakers because noun gender is arbitrary in many cases, but ejemplo is firmly masculine.

Not exactly.

Entender means to understand, not to explain.

So:

• ayuda a entender la definición = helps to understand the definition

If you wanted helps explain the definition, you would use explicar:

• ayuda a explicar la definición

That is an important distinction:

• entender = understand
• explicar = explain