En el índice del folleto no aparecía ese apartado, pero sí estaba en la página siguiente.

Because no aparecía describes a situation or state in the text rather than a single completed event.

Here, aparecía means something like was not listed / did not appear. The speaker is describing what the brochure’s index was like at that moment: that section wasn’t there.

Using the imperfect often gives this kind of background description:

• No aparecía en el índice = it wasn’t showing up / it wasn’t listed in the index

A preterite form like no apareció would sound more like a one-time event:

• No apareció = it didn’t appear

That could work in some contexts, but in this sentence the imperfect is more natural because it describes the content of the index.

Not literally in the visual sense. Here aparecer means:

• to appear
• to show up
• to be listed
• to be included

So no aparecía ese apartado means that section was not listed or did not appear in the index.

This is a very common use of aparecer in Spanish for text, lists, screens, records, and documents:

• Mi nombre no aparece en la lista.
• La dirección no aparece en el formulario.

Here apartado means section, heading, or entry within a text or document.

Depending on context, apartado can refer to:

• a section of a book or brochure
• a subsection
• a heading in a document
• a category or item in a list

So ese apartado is that section.

Be careful: apartado can mean other things in other contexts, such as P.O. box in apartado de correos, but that is not the meaning here.

Spanish demonstratives depend on how the speaker mentally positions the thing being talked about.

• este = this, closer to the speaker
• ese = that, often closer to the listener or just already identified but not especially close
• aquel = that over there, farther away

In this sentence, ese apartado refers to a specific section already known in the conversation or context, so ese is natural: that section.

A learner should not expect Spanish demonstratives to match English one-to-one in every case. Very often ese is simply the normal way to refer back to something already mentioned.

Here sí is an emphatic, contrastive yes / indeed / actually.

The sentence contrasts two ideas:

• it was not in the index
• but it was on the following page

So:

• pero sí estaba = but it was there / but it actually was

This sí is not just the basic answer yes. It is used to reinforce the contrast after a negative statement.

Compare:

• No estaba aquí, pero sí allí.
• No lo vi ayer, pero sí hoy.

It is very common after no... pero sí... patterns.

Because sí and si are different words.

• sí = yes, or an emphatic indeed / actually
• si = if

So in this sentence, it must be sí because it is emphasizing the contrast:

• pero sí estaba = but it was

Without the accent, si would mean if, which would make no sense here.

Because the meaning changes slightly.

• aparecer en el índice = to appear / be listed in the index
• estar en la página siguiente = to be on the following page

For the index, the idea is being listed. For the page, the idea is being located there.

So the sentence is very natural:

• no aparecía in the index
• sí estaba on the next page

You could not use ser here, because location uses estar, not ser.

Because estar is used for location.

The phrase en la página siguiente tells us where the section was located, so Spanish uses estar:

• estaba en la página siguiente = it was on the following page

Using ser would be incorrect here.

A basic rule:

• ser = identity, definition, essential characteristics
• estar = location, temporary states, resulting conditions

Here índice means index or table of contents, depending on context.

In Spanish, índice can refer to:

• an alphabetical index
• a contents list at the beginning of a book or booklet

In this sentence, because it is talking about a folleto and a section not appearing there, índice probably means something like the contents page or index section of the brochure.

So yes, it is close to English index, but in practice it can cover a slightly wider range depending on the type of publication.

Folleto usually means brochure, booklet, or pamphlet.

It refers to a small printed publication, often informational or promotional.

Depending on context, possible English translations include:

• brochure
• leaflet
• pamphlet
• booklet

So el índice del folleto means the brochure’s index / contents.

Because de + el contracts to del in Spanish.

So:

• de + el folleto → del folleto

This contraction is mandatory, except when el is part of a proper name:

• del libro
• del folleto
• but de El Escorial

So el índice del folleto simply means the index of the brochure.

That phrase is placed first to set the scene and highlight the contrast.

Spanish often puts a location or context at the beginning of the sentence:

• En el índice del folleto, no aparecía ese apartado...

This is similar to English:

• In the brochure’s index, that section didn’t appear...

It helps organize the information clearly:

1. first, where it was not found
2. then, where it was found

So the word order is very natural and gives the sentence a clear structure.

La página siguiente means the following page or the next page.

Yes, la siguiente página is also possible, but la página siguiente is very common and sounds very natural.

In Spanish, adjectives like siguiente can often go after the noun:

• la página siguiente
• el día siguiente
• la semana siguiente

This post-noun position is especially common in expressions meaning the next/following...

Yes, in many contexts that would be possible.

Spanish often omits repeated verbs when the meaning is obvious:

• En el índice del folleto no aparecía ese apartado, pero sí en la página siguiente.

This is shorter but still natural.

However, the full version pero sí estaba en la página siguiente is clearer and more explicit, especially for emphasis or careful writing.

Yes, no aparecía ese apartado is normal.

Spanish word order is more flexible than English. Both are possible:

• No aparecía ese apartado
• Ese apartado no aparecía

The version in the sentence puts the negative verb first after the opening phrase:

• En el índice del folleto no aparecía ese apartado

This sounds natural and keeps the focus on the fact that it wasn’t listed there.

If you said:

• Ese apartado no aparecía... the section itself becomes a little more prominent as the topic.

Both are correct; the difference is mainly one of emphasis and information flow.