Mi amiga escribió veintisiete líneas del resumen y luego acabó cambiando el final.

Escribió is the preterite, which is used for a completed action in the past. Here, the sentence presents writing veintisiete líneas as a finished event.

If you used escribía, it would sound more like an ongoing, repeated, or background action. For example:

• Mi amiga escribía mientras yo leía = My friend was writing while I was reading.

So in this sentence, escribió fits because the action is seen as completed.

Acabó cambiando means ended up changing.

This structure often suggests that the result was reached after some process, hesitation, or unexpected development. So it is not just she changed the ending, but more like she ended up changing the ending in the end.

Compare:

• Cambió el final = She changed the ending.
• Acabó cambiando el final = She ended up changing the ending.

The second one adds a sense of eventual outcome.

Because Spanish uses the structure acabar + gerund to mean to end up doing something.

So:

• acabó cambiando = ended up changing
• acabaron aceptándolo = they ended up accepting it
• acabé yendo = I ended up going

This is a very common structure, and it does not simply mean finished changing. In this sentence, it means the final result was that she changed the ending.

Because del is the normal contraction of de + el.

So:

• del resumen = de el resumen = of the summary

Spanish normally contracts de + el into del.

A similar contraction is:

• a + el = al

One important exception is when El is part of a proper name:

• de El Escorial
  not del Escorial

In standard Spanish, the numbers from 16 to 29 are usually written as one word:

• dieciséis
• veinte
• veintidós
• veintisiete

From 30 onward, they are usually written as separate words with y:

• treinta y uno
• cuarenta y siete

So veintisiete is the normal spelling.

Here luego means then or afterwards.

It shows the sequence of actions:

1. She wrote twenty-seven lines of the summary.
2. Then she ended up changing the ending.

In many contexts, luego is a very natural way to move the story forward.

Both words can relate to an ending, but they are not always used in the same way.

• el final usually means the ending of a story, film, book, essay, summary, etc.
• el fin can mean the end, but it can also mean purpose or goal.

In this sentence, el final is the natural choice because it refers to the ending of a text.

For example:

• No me gustó el final de la película = I did not like the ending of the film.
• El fin del curso = the end of the course

Because mi does not change for masculine or feminine in the singular.

So you get:

• mi amigo
• mi amiga

It only changes for number:

• mis amigos
• mis amigas

So mi works for both amigo and amiga.

Yes, Spanish could leave it out if the subject is already clear.

For example:

• Escribió veintisiete líneas del resumen y luego acabó cambiando el final.

That would still be grammatical. Spanish often drops subject pronouns and sometimes other subjects when context makes them clear.

However, Mi amiga is included here to make the sentence explicit and clear from the start about who did the actions.

Líneas has an accent mark because of its stress pattern and vowel combination.

It is pronounced roughly as:

• LÍ-ne-as

The accent on í helps show that í and e are pronounced in separate syllables rather than blending together.

By contrast, resumen does not need an accent mark because its stress already follows normal Spanish stress rules:

• re-SU-men

So the accent in líneas is necessary, while in resumen it is not.

It suggests she wrote part of the summary, specifically twenty-seven lines of it.

The phrase veintisiete líneas del resumen points to a quantity taken from a larger whole. So it sounds like the summary was longer, or at least that the sentence is focusing on one part of the writing process rather than saying she completed the whole summary.

If you wanted to say she wrote the whole summary, you might say something like:

• Mi amiga escribió el resumen = My friend wrote the summary

But here the sentence is more specific about how much she wrote.