Después de que pase el cartero, abriré la carta y la guardaré en el cajón.

Because this clause refers to a future action: the postman has not come by yet.

In Spanish, when después de que refers to something in the future, the verb that follows normally goes in the subjunctive:

• Después de que pase el cartero... = after the postman comes by

If it referred to something that had already happened, you would use the indicative instead:

• Después de que pasó el cartero... = after the postman came by
• Después de que pasa el cartero... can be used for habitual/general situations, but not for this one-time future event.

So here, pase is the present subjunctive of pasar.

Después de que means after when it is followed by a conjugated verb.

Compare:

• después de comer = after eating
• después de la comida = after the meal
• después de que pase el cartero = after the postman comes by

So:

• después de
  • noun / infinitive
• después de que
  • conjugated verb

The que is needed because what follows is a full clause: pase el cartero.

They are in the simple future because the speaker is talking about what they will do later:

• abriré = I will open
• guardaré = I will put away / keep / store

This matches the first part of the sentence, which sets up a future moment: Después de que pase el cartero...

In everyday spoken Spanish, people often use ir a + infinitive instead:

• Después de que pase el cartero, voy a abrir la carta y la voy a guardar en el cajón.

Both are correct. The simple future version sounds a bit more neutral or written.

Because la in la guardaré is a direct object pronoun that refers back to la carta.

So the structure is:

• abriré la carta = I will open the letter
• y la guardaré = and I will put it away

Spanish often avoids repeating the noun when it is already clear.

You could say:

• abriré la carta y guardaré la carta...

but that sounds repetitive, just like in English:

• I’ll open the letter and put the letter in the drawer

So la here simply means it.

Literally, yes, pasar often means to pass. But in this context, que pase el cartero is naturally understood as:

• when the postman comes by
• when the postman stops by
• when the mail is delivered

This is a very common use of pasar for someone coming around or passing by a place.

So although the literal idea is pass by, the natural meaning in English is closer to after the postman comes by.

Because guardar is usually used with en to show the place where something ends up being kept or stored:

• guardar algo en un cajón
• guardar la ropa en el armario
• guardar el dinero en el banco

So:

• la guardaré en el cajón = I’ll put it away in the drawer

Using a here would sound unnatural with guardar.

Even though there is movement involved, Spanish commonly uses en with verbs like poner, meter, guardar, etc. when talking about where something is placed.

Here guardar means something like:

• to put away
• to store
• to keep
• to put in a safe/place where it belongs

So la guardaré en el cajón is not just I will keep it, but more specifically I’ll put it away in the drawer.

It is a very useful verb in Spanish. For example:

• Guarda tus llaves en el bolso. = Keep/put your keys in the bag.
• Voy a guardar este documento. = I’m going to save/store this document.

Yes. Spanish allows some flexibility.

The original sentence is:

• Después de que pase el cartero, abriré la carta y la guardaré en el cajón.

You could also say:

• Abriré la carta y la guardaré en el cajón después de que pase el cartero.

Both are correct. The difference is mainly one of focus:

• putting Después de que pase el cartero first emphasizes the time condition first
• putting it at the end gives priority to the main action

The version with the subordinate clause first is very natural.

Because the sentence begins with a subordinate time clause:

• Después de que pase el cartero, ...

In Spanish, when this kind of clause comes before the main clause, a comma is normally used.

So:

• Después de que pase el cartero, abriré la carta...

But if the time clause comes at the end, the comma is usually not needed:

• Abriré la carta y la guardaré en el cajón después de que pase el cartero.

Yes, cartero is the masculine form.

If the postal worker is a woman, you can say:

• la cartera

However, learners should be careful, because cartera can also mean wallet or handbag, depending on context.

In this sentence, el cartero simply means the postman / mail carrier.

The written accent marks show the stressed syllable:

• abriré → a-bri-RÉ
• guardaré → guar-da-RÉ
• cajón → ca-JÓN

This is especially helpful because without the accent, the stress would normally fall elsewhere according to Spanish spelling rules.

Also, in Spain Spanish:

• j in cajón is pronounced like a strong throaty sound, similar to the ch in Scottish loch
• r in cartero is a tapped r
• ll does not appear here, but note that abriré and guardaré are regular future forms built from the infinitive + endings

Sometimes, yes, but it depends on the part of the sentence.

For the main clause, spoken Spanish often uses the present for future meaning if the context is clear:

• Después de que pase el cartero, abro la carta y la guardo en el cajón.

This can sound conversational, especially if the speaker is describing a plan.

But in your sentence, the future tense is perfectly natural and very clear.

For the clause after después de que, though, you still need the subjunctive if it refers to the future:

• Después de que pase... ✅
• Después de que pasa... ❌ for this future meaning

So the key rule is really about pase. The main clause has more flexibility.