En la aduana, la agente fue amable y nos explicó qué documentos faltaban.

En la aduana means at customs or literally in the customs area.

Spanish often begins with a place or time expression to set the scene. Here, En la aduana tells you where the action happened before introducing what happened there.

A few useful points:

• aduana = customs
• en = in / at
• la aduana is singular in Spanish, even though English often uses customs, which looks plural.

So:

• En la aduana = At customs

Agente is normally the same form for both masculine and feminine in standard Spanish. The gender is shown by the article:

• el agente = the male officer/agent
• la agente = the female officer/agent

So in this sentence, la agente tells you the agent was a woman.

You may occasionally hear agenta in some contexts, but la agente is the standard and safest form to learn.

This is a very common question because English just uses was.

Here, fue amable uses the preterite of ser:

• fue = was

In this sentence, the speaker is describing the agent’s behavior as part of a completed event at customs. It presents her kindness as part of what happened in that moment.

Why not the others?

• fue amable = she was nice / she acted nicely in that situation
• era amable = she was nice by nature / she used to be nice / she was being described in the background
• estaba amable = possible in some contexts, but much less natural here; it can suggest a temporary mood or manner

So fue amable fits best because the sentence tells a finished story about a specific interaction.

Nos explicó means she explained to us.

Breakdown:

• nos = to us / us
• explicó = explained

With explicar, Spanish often uses an indirect object pronoun for the person receiving the explanation:

• Me explicó = She explained to me
• Nos explicó = She explained to us
• Les explicó = She explained to them / you all

So nos is not the thing being explained. It tells you who received the explanation.

Explicó has an accent because it is the third person singular preterite form of explicar:

• yo expliqué
• tú explicaste
• él/ella/usted explicó

The accent shows the stress falls on the last syllable: ex-pli-CÓ.

This also helps distinguish it from explico:

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

So the accent is important for both pronunciation and meaning.

Here qué has an accent because it introduces an indirect question:

• nos explicó qué documentos faltaban
• she explained which documents were missing

Even though this is not a direct question with question marks, it still contains a question idea inside the sentence: which documents were missing?

Compare:

• qué = what / which in questions or indirect questions
• que = that, which, who, etc., with no question meaning

Examples:

• No sé qué quieres. = I don’t know what you want.
• Dijo que venía. = He said that he was coming.

Faltaban is the imperfect form of faltar:

• faltaban = were missing

This is very natural after explicó qué documentos... because the sentence is focusing on the state/situation of the documents at that time, not on a single completed action.

Compare:

• explicó qué documentos faltaban = she explained which documents were missing
• explicó qué documentos faltaron = would sound more like which documents were absent/failed to appear at some completed moment, which is not the usual idea here

In other words, faltaban describes an ongoing situation: those documents were missing from what we had.

In this sentence, faltar means to be missing or to be lacking.

So:

• qué documentos faltaban = which documents were missing

A useful thing to remember is that faltar often works differently from English. The thing that is missing becomes the subject:

• Falta un documento. = One document is missing.
• Faltan dos documentos. = Two documents are missing.

That is why the verb here is plural:

• documentos = plural
• faltaban = plural verb

Because the subject is documentos, which is plural.

Spanish makes the verb agree with the thing that is missing:

• Faltaba un documento. = One document was missing.
• Faltaban documentos. = Documents were missing.

In your sentence:

• qué documentos faltaban
• documentos is plural
• therefore faltaban must also be plural

Spanish does have that word: qué.

In indirect questions, qué can mean what or which, depending on context. Here it means which:

• qué documentos faltaban = which documents were missing

Spanish does not need a separate special word here in the same way English distinguishes between what documents and which documents. Qué covers that function.

The most natural option here is nos explicó.

Spanish object pronouns usually go before a conjugated verb:

• nos explicó
• me dijo
• les mostró

Other options:

• explicó a nosotros is not correct in standard Spanish
• If you want emphasis, you would say nos explicó a nosotros, but that is only for contrast or emphasis
• explicónos is not normal modern Spanish

So the standard pattern is:

• pronoun + conjugated verb
• nos explicó

Because Spanish often mixes tenses in narratives, and each tense has a different job.

Here:

• fue amable uses the preterite for a completed event in the story
• faltaban uses the imperfect for the background situation or ongoing state

So the sentence means something like:

• At customs, the agent was nice and explained to us
• which documents were missing

This combination is very common in Spanish storytelling:

• preterite for main completed actions
• imperfect for descriptions, states, or ongoing circumstances

Yes. The best translation depends on context.

Possible translations:

• the agent
• the officer
• the customs officer

In a customs context, English often sounds more natural with officer or customs officer, even though the Spanish word is agente.

So:

• la agente fue amable could be translated as
  • the officer was kind
  • the customs officer was nice
  • the agent was nice

All are possible; some just sound more natural in English.

It is not always strictly necessary, but it is very natural.

The comma separates the opening location phrase from the main clause:

• En la aduana, la agente fue amable...

It helps readability and signals a pause, like:

• At customs, the agent was kind...

Without the comma, the sentence is still understandable:

• En la aduana la agente fue amable...

But the version with the comma is clearer and more polished in writing.