En el mostrador, la mensajera me explicó que faltaba un sello en el paquete.

En el mostrador means at the counter.

Here en is used because it describes the location where the explanation happened. The speaker is saying they were at the counter.

• en = in / on / at, depending on context
• a usually expresses movement toward a place

So:

• En el mostrador = At the counter
• Voy al mostrador = I’m going to the counter

Mostrador usually means counter, especially in places like:

• a post office
• a shop
• an airport check-in area
• a reception desk

In this sentence, it sounds like the speaker is at some kind of service counter, probably in a post office or shipping office.

La mensajera means the female courier, the female delivery worker, or the female messenger, depending on context.

It is the feminine form of:

• el mensajero = male courier / messenger
• la mensajera = female courier / messenger

Because the noun refers to a woman, Spanish uses the feminine article la and the feminine ending -a.

Me means to me.

So:

• explicó = explained
• me explicó = explained to me

Spanish often uses an indirect object pronoun where English may or may not say it explicitly.

Examples:

• Me explicó la situación = She explained the situation to me
• Nos explicó el problema = She explained the problem to us

Explicó is the preterite, which is used for a completed action in the past.

So la mensajera me explicó presents the explanation as a finished event:

• The courier explained to me...

If you used explicaba, that would sound more like:

• she was explaining
• she used to explain
• it was background information or an ongoing action

Here the sentence is about one completed moment, so explicó fits naturally.

This is a very common question. In me explicó que faltaba un sello, the imperfect faltaba is used because it describes the situation that existed at that moment: a stamp was missing.

It is not really focusing on the moment the stamp became missing, but on the state/problem.

Compare:

• Faltaba un sello = A stamp was missing
• Faltó un sello = more like A stamp was lacking / one stamp was absent, often sounding more event-like or less natural here

After verbs like explicar, decir, ver, or notar, Spanish often uses the imperfect to describe the situation someone noticed or explained.

Faltar often works differently from English to miss.

Here, faltaba un sello literally means something like:

• a stamp was lacking
• a stamp was missing

The thing that is absent becomes the subject of the verb:

• Faltaba un sello = A stamp was missing
• Faltaban dos documentos = Two documents were missing

So un sello is the subject, which is why the verb is singular: faltaba.

Here sello means stamp.

Given the context of a package and a counter, this probably means a postal or shipping stamp/label required for sending the package.

Be aware that sello can also mean:

• seal
• stamp in other contexts
• sometimes a rubber stamp impression

But in this sentence, stamp is the most natural meaning.

En el paquete means on/in the package, depending on how the speaker sees it.

In context, faltaba un sello en el paquete usually means:

• the package did not have a required stamp on it

Spanish often uses en in cases where English might say on.

So although English says on the package, Spanish may say en el paquete.

If you said del paquete, that would mean of the package or from the package, which is a different idea.

Yes, absolutely. That is a very natural English translation.

Spanish and English often structure this idea differently.

Spanish:

• faltaba un sello en el paquete

Natural English options:

• There was a stamp missing from the package
• The package was missing a stamp
• A stamp was missing from the package

All of these capture the idea well.

Putting En el mostrador first sets the scene: it tells you where the action happened before giving the main event.

This kind of word order is very normal in Spanish. It can sound slightly more narrative or descriptive:

• En el mostrador, la mensajera me explicó...
• At the counter, the courier explained to me...

You could also say:

• La mensajera me explicó en el mostrador que faltaba un sello en el paquete

That is also grammatical, but the original sentence flows nicely because it establishes the setting first.

Yes. Here que means that and introduces the content of what was explained.

• Me explicó que... = She explained to me that...

This is extremely common in Spanish after verbs like:

• decir = to say
• explicar = to explain
• creer = to believe
• pensar = to think

Yes. Mensajera can mean:

• messenger
• courier
• sometimes someone who delivers messages or packages

The exact translation depends on context. Because the sentence mentions the counter, a stamp, and the package, courier or postal worker makes the most sense here.