Mañana pasaré por la oficina de correos antes de volver a casa.

Pasaré is the simple future of pasar, meaning I will stop by / I will go by.

So:

• Mañana pasaré... = Tomorrow I’ll stop by...

In Spanish, the future tense is very natural for planned actions in the future. However, just like in English, Spanish can also often use the present for future plans:

• Mañana paso por la oficina de correos...

That sounds more conversational in many contexts. Pasaré can sound a bit more definite, neutral, or formal.

Here, pasar por means to go by, to stop by, or to drop in at/on the way past.

It does not literally mean just to pass through in this context. The idea is that the speaker will go to the post office briefly, probably as one step in their journey before going home.

Examples:

• Paso por tu casa luego. = I’ll stop by your house later.
• Pasé por el banco esta mañana. = I stopped by the bank this morning.

So in this sentence, pasaré por la oficina de correos is best understood as I’ll stop by the post office.

Spanish often uses multi-word expressions where English uses a single noun.

• la oficina de correos literally means the office of mail/post
• natural meaning: the post office

You may also hear:

• Correos in Spain, especially when referring to the postal service or the building in context
• la oficina postal, though this is less common in everyday speech than la oficina de correos

For a learner, la oficina de correos is a very safe and standard phrase.

Because Spanish usually drops subject pronouns when the verb already makes the subject clear.

• pasaré already means I will stop by
• so yo is unnecessary unless you want emphasis or contrast

For example:

• Mañana pasaré por la oficina de correos. = normal
• Yo pasaré por la oficina de correos, pero tú no. = I’ll stop by the post office, but you won’t.

In most everyday sentences, leaving out yo sounds more natural.

Because in Spanish, antes de + infinitive is the normal structure for before doing something.

So:

• antes de volver = before returning / before going back

Compare:

• antes de comer = before eating
• antes de salir = before leaving

If a full clause follows, Spanish uses antes de que + subjunctive:

• antes de que vuelva a casa = before I return home

But here the subject is the same person, so antes de + infinitive is the natural choice.

Yes, volver means to return or to go back, and that is exactly the idea here.

• volver a casa = to go back home / to return home

In English, we often simply say go home, but Spanish commonly uses volver a casa when the meaning is return home rather than just go home in a general sense.

Another common option is:

• ir a casa = to go home
• volver a casa = to go back home / return home

So volver adds the sense of returning.

In Spanish, casa often appears without an article when it means home in a general, personal sense.

So:

• volver a casa = to return home
• llegar a casa = to arrive home
• estar en casa = to be at home

But if you mean a specific house as a building, then you may use the article:

• Volví a la casa de mi abuela. = I went back to my grandmother’s house.

So in your sentence, a casa means home, not just to the house.

Usually not if you specifically mean the post office building.

In Spain:

• Correos often refers to the postal service
• la oficina de correos refers clearly to the post office branch/location

El correo more often means mail/post itself, not the building.

Examples:

• He recibido el correo. = I’ve received the mail.
• Voy a la oficina de correos. = I’m going to the post office.

So for this sentence, la oficina de correos is the best choice.

Yes, but the meaning changes slightly.

• Pasaré por la oficina de correos = I’ll stop by the post office
• Iré a la oficina de correos = I’ll go to the post office

Pasaré por suggests it is a quick visit, perhaps on the way somewhere else. Since the sentence continues with antes de volver a casa, that fits very well: the speaker will stop there before heading home.

Iré a is more neutral and only states the destination.

Mañana means tomorrow, and here it sets the time frame for the whole sentence.

Spanish often places time expressions at the start of a sentence:

• Mañana pasaré por la oficina de correos...
• Hoy no trabajo.
• El lunes salimos temprano.

This is very natural and similar to English starting with Tomorrow...

Also, note the difference between:

• mañana = tomorrow
• la mañana = the morning

So context matters.

Yes, Spanish word order is fairly flexible, although some versions sound more natural than others.

Original:

• Mañana pasaré por la oficina de correos antes de volver a casa.

Other possible orders:

• Pasaré por la oficina de correos mañana antes de volver a casa.
• Antes de volver a casa, pasaré por la oficina de correos mañana.

The first version is the most natural and straightforward. Putting mañana first is very common because it immediately tells you when the action will happen.

A very natural casual version could be:

• Mañana paso por Correos antes de volver a casa.

Why?

• present tense for a planned future action: paso
• Correos used as a familiar shorthand in Spain

Another natural option:

• Mañana voy a pasar por la oficina de correos antes de volver a casa.

This uses ir a + infinitive, which is also very common in speech.

So the original sentence is perfectly correct, but everyday spoken Spanish may prefer slightly different forms depending on tone and region.