Antes de entrar en la terminal, una agente nos explicó cómo pasar por la aduana.

Because antes de + infinitive is the normal way to say before doing something.

So:

• antes de entrar = before entering

If Spanish needs to mention a different subject, it usually switches to antes de que + subjunctive:

• Antes de que entráramos en la terminal... = Before we entered the terminal...

In your sentence, the infinitive works because the subject is understood in a general way.

In Spain, entrar en is the most standard and expected choice for entering a place.

• entrar en la terminal
• entrar en el edificio
• entrar en casa

You may hear entrar a in some parts of Latin America, but for Spanish from Spain, entrar en is the safest option.

Agente is usually a common-gender noun, which means the word itself often stays the same, and the article shows whether the person is male or female:

• un agente = a male agent
• una agente = a female agent

So una agente is completely normal and natural.

Nos means to us here.

The verb pattern is:

• explicar algo a alguien = to explain something to someone

So:

• una agente nos explicó cómo pasar por la aduana
• literally: an agent explained to us how to go through customs

Even though English often just says explained, Spanish commonly includes the person receiving the explanation with an indirect object pronoun like me, te, le, nos, os, les.

Explicó is the preterite form of explicar, third person singular:

• yo expliqué
• tú explicaste
• él/ella explicó

The preterite is used because this is a completed action in the past: the agent explained it, and that event is finished.

If it were background information or something habitual, Spanish might use the imperfect instead, but here it is a single completed event.

The accent mark shows the correct stress and also distinguishes it from another form:

• explico = I explain
• explicó = he/she explained

So the accent is important both for pronunciation and meaning.

Because cómo here is an interrogative word in an indirect question.

After verbs like explicar, decir, saber, preguntar, Spanish often uses words like:

• cómo = how
• qué = what
• cuándo = when
• dónde = where

So:

• nos explicó cómo pasar por la aduana

means she explained how to go through customs.

That is why cómo keeps its written accent.

Because Spanish often uses interrogative word + infinitive to mean how to do something, what to do, where to go, and so on.

So:

• cómo pasar por la aduana = how to go through customs

This is very common and natural.

A more expanded version could be something like:

• nos explicó cómo teníamos que pasar por la aduana

But the infinitive version is shorter and very idiomatic.

Here por helps create the idea of going through a place or process.

So:

• pasar por la aduana = to go through customs

It does not mean just walking near customs. It means going through that checkpoint or procedure.

This use of por is common with movement through places or stages:

• pasar por la puerta = go through the door
• pasar por seguridad = go through security

Because Spanish uses aduana as a singular noun, even though English normally says customs.

So:

• la aduana = customs

This is just a vocabulary difference between the two languages. English uses a plural-looking word, but Spanish uses a singular one.

Because Antes de entrar en la terminal is an introductory phrase placed at the beginning of the sentence.

Spanish often separates this kind of opening phrase with a comma, especially when it is a bit longer:

• Antes de entrar en la terminal, una agente nos explicó...

It helps mark the pause and makes the sentence easier to read.

Spanish uses articles more often than English, especially with specific places or nouns that are clearly identified in context.

So:

• en la terminal
• por la aduana

sound natural because the speaker has a particular terminal and the customs area in mind. English might sometimes omit or handle articles differently, but in Spanish they are very often required.