Mi hermana escribió el quinto apartado del informe y luego revisó el índice.

Mi without an accent is the possessive adjective meaning my.

Mí with an accent is a pronoun used after prepositions, for example:

• para mí = for me
• sin mí = without me

Also, Spanish normally does not use an article before these short possessives. So:

• mi hermana = my sister
• not la mi hermana in standard modern Spanish

Spanish often leaves out subject pronouns when the subject is already clear. This is called being a pro-drop language.

Here, mi hermana already tells you who did the actions, so there is no need to say ella as well:

• Mi hermana escribió... y luego revisó...

Adding ella would usually sound emphatic or contrastive:

• Ella escribió el apartado, no yo.

They are both third-person singular preterite forms:

• escribió = he/she wrote
• revisó = he/she reviewed/checked

The written accent shows the correct stress and also helps distinguish forms. For example:

• reviso = I review/check (present)
• revisó = he/she reviewed/checked (preterite)

So the accent is an important part of the spelling.

The preterite is used for completed actions, especially when they happened in sequence.

That is exactly what the sentence describes:

1. she wrote the fifth section
2. then she reviewed the index

So:

• escribió = she wrote
• revisó = she reviewed/checked

If the sentence were describing repeated or ongoing past actions, the imperfect would be more likely:

• Mi hermana escribía informes... = My sister used to write reports...

In this context, apartado means something like section, part, or heading/division of a document.

It does not usually mean paragraph. A paragraph is more specifically:

• párrafo

So in a report, apartado suggests a numbered or titled section of the text.

Quinto is an ordinal number meaning fifth.

It appears as quinto because it agrees with apartado, which is:

• masculine
• singular

So:

• el quinto apartado = the fifth section

If the noun were feminine, the form would change:

• la quinta sección

Ordinals usually come before the noun in normal modern Spanish:

• el quinto capítulo
• la tercera parte

Putting the ordinal after the noun can sound more formal, literary, or old-fashioned in some contexts.

Because de + el contracts to del in Spanish.

So:

• de el informe becomes del informe

This contraction is mandatory in normal Spanish.

Examples:

• la portada del libro
• el final del informe

A common exception is when El is part of a proper name:

• de El Escorial

Índice can mean several related things, including:

• index
• table of contents
• list
• indicator in other contexts

In a sentence about a report, el índice will often mean the table of contents.

So the exact English translation depends on context, but in document-related Spanish, índice is very commonly used for the contents page or contents list.

Not always. Revisar often means:

• to review
• to check
• to go over
• to inspect

So revisó el índice often means she checked or reviewed the index/table of contents.

Sometimes English learners assume revisar always means to revise, but that can be misleading. In many contexts, check/review is more natural.

They do slightly different jobs:

• y = and → joins the two clauses
• luego = then / afterwards → shows the order of events

So:

• y luego = and then

You could say:

• Mi hermana escribió el quinto apartado del informe y revisó el índice.

That is still correct, but luego makes the sequence clearer.

Because the direct objects here are things, not people:

• el quinto apartado
• el índice

The personal a is usually used before a specific person (or sometimes a pet treated like a person) when that person is the direct object:

• Vi a mi hermana. = I saw my sister.

But with things, you normally do not use it:

• Escribió el apartado
• Revisó el índice