Tengo que pasar por la oficina de correos antes de que cierre.

Tengo que + infinitive means I have to ... or I must ....

So:

• Tengo que pasar = I have to stop by / I have to go by

This is one of the most common ways to express obligation in Spanish.

Examples:

• Tengo que estudiar. = I have to study.
• Tenemos que salir. = We have to leave.

Here, pasar por does not usually mean literally pass through. In this context, it means:

• to stop by
• to go by
• to drop in at

So pasar por la oficina de correos means something like stop by the post office.

It often suggests a brief visit, or going there as part of your errands.

Compare:

• Ir a la oficina de correos = to go to the post office
• Pasar por la oficina de correos = to stop by the post office

Yes. La oficina de correos means the post office.

Literally, it is something like the office of mail/post, but in natural English it is simply the post office.

In Spain, you may also hear:

• Correos = the postal service / the post office
• la oficina de Correos = the post office branch

So in Spain, a very natural shorter version would be:

• Tengo que pasar por Correos antes de que cierre.

Because antes de que is used when it is followed by a conjugated verb.

Here we have:

• antes de que cierre

Since cierre is a full verb form, you need que.

Use:

• antes de que + verb

Examples:

• antes de que llegue = before he/she arrives
• antes de que empiece = before it starts

By contrast, antes de is used before a noun or an infinitive:

• antes de salir = before leaving
• antes del almuerzo = before lunch

Because after antes de que, Spanish normally uses the subjunctive when talking about something that has not happened yet.

So:

• cierra = present indicative
• cierre = present subjunctive

In this sentence, the closing is still in the future from the speaker’s point of view, so Spanish uses the subjunctive:

• antes de que cierre = before it closes

This is very common in Spanish:

• antes de que llegue
• antes de que sea tarde
• antes de que empiece la película

The implied subject is la oficina de correos.

So antes de que cierre means:

• before it closes
• before the post office closes

Spanish often leaves out the subject when it is clear from the context.

If you wanted to make it explicit, you could say:

• Tengo que pasar por la oficina de correos antes de que la oficina cierre.

But that sounds more repetitive. The original sentence is more natural.

Not with exactly the same meaning.

• antes de que cierre = before it closes / before the post office closes
• antes de cerrar = before closing

The second version is more ambiguous, because it does not clearly say who is closing. It could sound like:

• before I close
• before they close
• before closing in general

So if you specifically mean before the post office closes, antes de que cierre is the clearer and more natural choice.

Yes, it is natural and completely correct in Spain.

That said, in everyday Spanish in Spain, people might often shorten it to:

• Tengo que pasar por Correos antes de que cierre.

That sounds very natural because Correos is the name of the Spanish postal service and is commonly used to mean the post office.

You might also hear:

• Tengo que ir a Correos antes de que cierre.

The difference is:

• ir a = go to
• pasar por = stop by

Spanish often drops subject pronouns because the verb form already tells you who the subject is, or the context makes it obvious.

In this sentence:

• Tengo already tells you the subject is I
• cierre refers back to la oficina de correos

So Spanish does not need:

• yo tengo
• ella / eso cierre

This is very normal in Spanish.

You could say Yo tengo que... for emphasis, but normally it is unnecessary.

Yes, but the nuance is slightly different.

• Tengo que pasar por la oficina de correos... = I have to stop by the post office...
• Debo pasar por la oficina de correos... = I should / I ought to / I must stop by the post office...

In many contexts, tener que sounds more like a practical necessity or obligation.
Deber can sometimes sound a bit more formal, or like a duty, recommendation, or stronger obligation depending on context.

For everyday errands, tengo que is often the most natural choice.