Después de una despedida larga, mi amiga subió al avión.

Because después needs de when it is followed by a noun or noun phrase.

• después de una despedida larga = after a long goodbye
• después on its own = afterwards / later

You can also get:

• después de + noun → después de la cena
• después de + infinitive → después de comer
• después de que + verb → después de que llegó

So in this sentence, de is required because una despedida larga is a noun phrase.

Despedida is a noun meaning farewell, goodbye, or parting.

It comes from the verb despedir or despedirse:

• despedirse = to say goodbye
• una despedida = a goodbye / farewell

So una despedida larga is literally a long farewell or a long goodbye.

It is a feminine noun, which is why it uses:

• una despedida
• despedida larga

In Spanish, descriptive adjectives usually come after the noun.

So:

• una despedida larga = a long goodbye

That is the most neutral, standard order.

You can also say una larga despedida, but that sounds a bit more literary or expressive. Putting the adjective before the noun often adds emphasis or a more subjective tone.

So both are possible, but una despedida larga is the straightforward choice.

No. Mi stays the same for both masculine and feminine singular nouns.

So:

• mi amigo = my male friend
• mi amiga = my female friend

The noun changes, not mi.

For plural:

• mis amigos
• mis amigas

Al is the contraction of a + el.

So:

• a el avión is incorrect
• al avión is correct

This contraction is mandatory in Spanish whenever a and el come together, except with the proper name El in names like a El Escorial.

Here:

• subió al avión = she got onto the plane / boarded the plane

Because subir al avión specifically means to get on or to board the plane.

Compare:

• fue al avión = she went to the plane
• subió al avión = she got on the plane

So subió focuses on the act of boarding, not just moving toward it.

This is a very common Spanish way to express getting onto transport:

• subir al tren
• subir al autobús
• subir al avión

Yes. Subió al avión and se subió al avión are both possible.

In many contexts:

• subió al avión = she boarded the plane
• se subió al avión = she got herself onto the plane / she boarded the plane

The version with se can sound a bit more explicit or dynamic, but in everyday use both can work.

In this sentence, subió al avión is perfectly natural.

Because the sentence describes a completed action in the past.

• subió = she boarded / she got on

The preterite is used for actions seen as finished events in a story.

If you used the imperfect:

• subía al avión

that would suggest something ongoing, repeated, or backgrounded, such as:

• Cuando la vi por última vez, subía al avión = When I saw her for the last time, she was boarding the plane

So subió is the normal choice for a single completed event.

The accent mark shows the correct stress.

• subió is stressed on the last syllable: su-bi-ó

This is the third-person singular preterite form of subir:

• yo subí
• tú subiste
• él/ella subió

Many preterite forms like this have a written accent:

• comió
• vivió
• salió

The accent helps show both pronunciation and the correct verb form.

That opening part is an introductory time phrase.

Spanish often uses a comma after a longer introductory phrase:

• Después de una despedida larga, mi amiga subió al avión.

The comma helps separate the background information from the main action.

In very short phrases, the comma is sometimes omitted, but here it is natural and clear.

Because possessive adjectives like mi, tu, su, nuestro, etc. normally replace the article.

So you say:

• mi amiga not
• la mi amiga

This works the same way with many nouns:

• mi casa
• mi hermano
• mis libros

So mi amiga is the normal standard form.

Yes. A few natural alternatives are:

• Tras una despedida larga, mi amiga subió al avión.
• Después de despedirse largamente, mi amiga subió al avión.
• Después de una larga despedida, mi amiga subió al avión.

These all express a very similar idea, but the original sentence is perfectly natural and clear.