Antes de despedirnos, mi amiga metió el clip y el sacapuntas en mi estuche.

Antes de means before.

In Spanish, when before is followed by a verb in its basic form, you usually use antes de + infinitive:

• Antes de salir = before leaving
• Antes de comer = before eating
• Antes de despedirnos = before saying goodbye

So here, de is required because the structure is antes de + infinitive.

Because the verb is despedirse, not just despedir.

• despedir = to dismiss, to fire, to send off
• despedirse = to say goodbye

In this sentence, the meaning is to say goodbye, so the reflexive form is needed.

The ending -nos means ourselves / we. So:

• despedirnos = for us to say goodbye / saying goodbye

Because after antes de, if the subject stays the same, Spanish normally uses an infinitive.

So:

• Antes de despedirnos... = before saying goodbye...

This is more natural than using a full conjugated clause when the people involved are the same.

If the subject changes, Spanish often uses antes de que + subjunctive instead:

• Antes de que ella se despidiera... = before she said goodbye

But in your sentence, antes de despedirnos is the normal structure.

Nos means us and shows that the goodbye involved we/us.

So despedirnos suggests before we said goodbye or before saying goodbye.

It does not have to mean that we said goodbye to each other only. It can also mean we said goodbye in a more general sense, depending on context.

Metió is the preterite tense, third person singular, from meter.

• yo metí
• tú metiste
• él/ella metió

It is used because the action is seen as a completed event in the past: your friend put the items into the pencil case, and that action is finished.

In English, this is often translated with the simple past:

• my friend put...

Here, meter means to put in, to place inside, or to stick into something.

So:

• metió el clip y el sacapuntas en mi estuche = she put the paper clip and the sharpener into my pencil case

It is similar to poner, but meter often emphasizes putting something inside a space or container.

Compare:

• poner el libro en la mesa = to put the book on the table
• meter el libro en la mochila = to put the book into the backpack

So meter fits very naturally with en mi estuche.

Because in Spanish, it is very common to repeat the article before each noun in a list, especially when the nouns are singular count nouns.

So this sounds natural:

• el clip y el sacapuntas

It would sound less standard to leave out the second article here.

Repeating the article makes the structure clear and natural.

Clip is normally treated as a masculine noun in Spanish, so it takes el:

• el clip
• los clips

This is simply the conventional gender the noun has in Spanish. Borrowed words still get assigned a grammatical gender.

In Spain, clip is a normal everyday word for paper clip.

Because sacapuntas is one singular noun, even though it ends in -s.

It is a compound word:

• saca- from sacar
• puntas = points/tips

Together, it means pencil sharpener.

Many learners expect a final -s to mean plural, but that is not always true. Here:

• el sacapuntas = the pencil sharpener
• los sacapuntas = the pencil sharpeners

So the singular and plural often look the same except for the article.

In this context, estuche most naturally means pencil case in Spain.

More generally, estuche can mean any kind of case or container for holding objects, depending on context:

• estuche de lápices = pencil case
• estuche de gafas = glasses case
• estuche de violín = violin case

Since the sentence mentions a clip and a sacapuntas, pencil case is the most likely meaning here.

Because Spanish uses en for movement into or location inside a container in expressions like this.

• meter algo en algo = to put something into something

So:

• metió el clip en mi estuche = she put the paper clip into my pencil case

Using a would not be correct here.

Yes. Dentro de mi estuche is possible and means inside my pencil case.

But en mi estuche is the most natural and simple option here.

Compare:

• metió el clip en mi estuche = normal, everyday
• metió el clip dentro de mi estuche = also correct, slightly more explicit

The accent mark shows the correct stress.

• metió is stressed on the last syllable: me-tió

It also helps distinguish the preterite form from other possible patterns.

This ending is very common in third person singular preterite forms of -er and -ir verbs:

• comió
• vivió
• metió

Yes, but it would be a different structure.

• Antes de despedirnos... = before saying goodbye...
• Antes de que nos despidiéramos... = before we said goodbye...

Both can work, but antes de + infinitive is especially common when the subject stays the same and the sentence can be expressed more simply.

So in your sentence, Antes de despedirnos is the more straightforward and natural choice.