En la última página, mi amiga añadió el índice y subrayó cada apartado con un subrayador amarillo.

Because en is the normal preposition for location on/in a page in Spanish: en la página, en la última página, en la primera línea, etc.

A native English speaker may expect something closer to on the last page, and that is exactly how Spanish commonly expresses it here with en.

• en la última página = on the last page
• not usually a la última página here, because a would suggest movement toward a page rather than location on it

Fronting this phrase also helps set the scene first: On the last page, ...

They are both in the preterite tense, third-person singular, because the sentence describes completed actions in the past.

• añadió = she added
• subrayó = she underlined

The subject is mi amiga, so the verb has to match she.

This tense is used because the actions are presented as finished events:

• she added the index
• she underlined each section

If the sentence were describing habitual or background actions, Spanish might use the imperfect instead, but here the preterite is the natural choice.

They have accent marks for standard Spanish spelling reasons.

• añadió and subrayó: the accent marks show the stress falls on the final syllable, as expected in these preterite forms
• última: the stress is on the first syllable, which needs an accent because otherwise the word would not follow the default stress pattern
• índice: the stress is also on the first syllable, so it needs an accent mark

For a learner, the most useful thing is simply to learn these as the correct written forms:

• última
• índice
• añadió
• subrayó

Also note the letter ñ in añadió. It is a separate letter in Spanish and is not the same as n.

Spanish often uses the definite article where English may omit it.

Here, el índice refers to a specific thing that was added: the index or the table of contents, depending on context.

In Spanish, it is very natural to say:

• añadió el índice
• corrigió el título
• cambió la portada

Even when English might sometimes say added an index or simply added index information, Spanish usually wants the article with a concrete noun like this.

Índice can mean different but related things depending on context:

• table of contents
• index
• sometimes a list or indicator in other contexts

In a sentence about a page and document structure, el índice most likely means the table of contents or possibly the index, depending on what the larger text is about.

In Spain, índice is commonly used for the table of contents of a book or document.

Here, apartado means something like:

• section
• heading
• subsection
• entry

It is a common word in Spanish for a division within a text, form, document, or set of instructions.

So cada apartado means each section or each heading/entry, depending on the context.

Because cada is followed by a singular noun in Spanish.

So:

• cada apartado = each section
• cada página = each page
• cada día = each day

Even though the meaning is distributive and refers to multiple items overall, the noun after cada stays singular.

This is a very common pattern that English speakers need to get used to.

Con is used to express the instrument or tool used to do something.

So:

• subrayó cada apartado con un subrayador amarillo = she underlined each section with a yellow highlighter/underliner

This is the same pattern as:

• escribió con un bolígrafo = she wrote with a pen
• cortó el papel con unas tijeras = she cut the paper with scissors

Here, amarillo agrees with subrayador, which is masculine singular:

• un subrayador amarillo

It is understandable, but it is not the only possible word, and in everyday use other terms may also be common depending on the region and speaker.

A few possibilities are:

• subrayador
• resaltador
• marcador
• rotulador fluorescente

In Spain, many speakers would also say something like marcador fluorescente or subrayador depending on habit and region.

So the sentence is perfectly understandable, but vocabulary for stationery can vary quite a bit across the Spanish-speaking world.

Possibly, but the meaning changes slightly.

• subrayar specifically means to underline
• resaltar often means to highlight
• marcar is more general: to mark

If the text literally has a line drawn under each section, subrayó is the best choice.

If the idea is that she highlighted the sections with a yellow marker, resaltó could also work in some contexts. Since the sentence includes un subrayador amarillo, it suggests a highlighting/underlining tool, but subrayó still clearly means the action was underlining.

Yes, Spanish can often omit subject pronouns such as ella, but here mi amiga is not just a pronoun—it is a full noun phrase, and it adds clear information about who did the actions.

You could say:

• Añadió el índice y subrayó cada apartado...

That would still be grammatical if the context already makes it clear who she is.

But using mi amiga is perfectly natural when you want to identify the person explicitly.

Yes. Spanish word order is flexible.

This version:

• En la última página, mi amiga añadió el índice y subrayó cada apartado con un subrayador amarillo.

puts the location first, which helps frame the action.

Other possible orders include:

• Mi amiga añadió el índice y subrayó cada apartado con un subrayador amarillo en la última página.
• Mi amiga, en la última página, añadió el índice...

The original order sounds natural because it begins with the setting and then gives the actions.

Yes, but the nuance changes.

• añadió = added
• agregó = added, but in Spain it can sound a bit less common or slightly more formal/neutral depending on context
• puso = put/placed, which is more general and less precise

For a document or text, añadir is a very good verb because it clearly expresses that something was inserted or included as extra content.

So añadió el índice is a strong, natural choice.