En el mostrador de correos, la empleada me dijo que faltaba otro sello.

This opening phrase sets the scene: it tells you where the action happened.

• en = at / in
• el mostrador = the counter
• de correos = of the post office / postal service

So En el mostrador de correos means something like At the post office counter.

Spanish often puts a place phrase at the beginning of the sentence to provide context before the main action.

In Spain, Correos is the usual name of the postal service, and the word is commonly used in the plural.

So:

• correos can refer to the post office / postal service
• mostrador de correos = post office counter

This is just how the institution is commonly referred to. It is similar to how some place names or services do not translate word-for-word in the most obvious way.

Mostrador means counter or service desk.

At a post office, you might also hear:

• la ventanilla = the service window
• el mostrador = the counter

They are similar, but not always identical:

• ventanilla suggests a window where a clerk serves you
• mostrador suggests a counter or desk surface

In many contexts, either could work depending on the exact layout of the post office.

La empleada means the female employee.

It is feminine because the worker being referred to is a woman:

• el empleado = male employee
• la empleada = female employee

In real life, depending on context, Spanish speakers might also say:

• la trabajadora = the female worker
• la dependienta = the female shop assistant / clerk
• la empleada de correos = the post office employee

Here, la empleada is a straightforward and natural way to say the female employee.

Me is the indirect object pronoun meaning to me.

So:

• dijo = said
• me dijo = said to me / told me

Spanish often uses an indirect object pronoun where English might simply say told me.

Examples:

• Me dijo la verdad. = She told me the truth.
• Me dijo que no. = She told me no.

So la empleada me dijo means the employee told me.

Because decir que normally introduces a reported statement, and reported statements usually take the indicative, not the subjunctive.

So:

• me dijo que faltaba otro sello = she told me that another stamp was missing

The speaker is reporting what the employee said as information, not as a wish, doubt, command, or emotion.

Compare:

• Me dijo que faltaba otro sello. = She told me that another stamp was missing. → indicative
• Me pidió que volviera mañana. = She asked me to come back tomorrow. → subjunctive, because pedir que expresses a request

Faltaba is the imperfect of faltar.

Here it describes a situation that was true at that moment: another stamp was still missing / needed. The imperfect is natural for describing a state or ongoing circumstance in the past.

• faltaba = was missing / was needed
• faltó = was missing at a specific completed moment / ended up missing

In this sentence, the idea is not really a single completed event, but a condition the employee noticed and reported.

So faltaba otro sello feels like:

• another stamp was missing
• one more stamp was needed

That is why the imperfect works well here.

This is a very common point for English speakers.

Spanish often uses faltar from the perspective of what is missing, not who needs it.

So:

• faltaba otro sello literally = another stamp was missing
• natural English meaning = I needed another stamp

The grammatical subject in Spanish is otro sello.

You can think of it like this:

• English: I was missing another stamp
• Spanish: Another stamp was missing

Spanish can also make the person explicit:

• me faltaba otro sello = I was missing another stamp / I still needed another stamp

In your sentence, the person affected is understood from the context, so me is only attached to dijo, not to faltaba.

Because otro normally does not take un before it.

So you say:

• otro sello = another stamp
• otra carta = another letter

Not:

• un otro sello ❌

This is one of those fixed grammar points in Spanish: otro already carries the idea of another / one more, so un is unnecessary.

Here, sello means stamp, specifically a postage stamp.

It can also mean seal or official stamp in other contexts, so the exact meaning depends on the situation.

In a post office sentence like this, sello is naturally understood as postage stamp.

Yes, absolutely.

• La empleada me dijo que faltaba otro sello.
• La empleada me dijo que me faltaba otro sello.

Both are possible.

The version with me faltaba makes it clearer that I was the one who still needed another stamp.

The version without me is a bit more neutral and focuses on the missing item itself: another stamp was missing.

So:

• faltaba otro sello = another stamp was missing
• me faltaba otro sello = I was missing another stamp / I still needed another stamp

Yes, you could.

Spanish word order is flexible, especially with time and place expressions.

These are both fine:

• En el mostrador de correos, la empleada me dijo que faltaba otro sello.
• La empleada me dijo en el mostrador de correos que faltaba otro sello.

Starting with En el mostrador de correos gives the sentence a scene-setting feel: At the post office counter...

Putting it later sounds slightly more neutral and less descriptive.

It is very natural here because the sentence begins with a fairly long introductory place phrase.

• En el mostrador de correos, la empleada me dijo...

The comma helps separate the setting from the main clause.

In shorter phrases, Spanish sometimes omits the comma more easily, but here the comma improves clarity and rhythm. So it is a good, natural choice.

Yes, it sounds natural and understandable in Spain.

A Spaniard might also say similar things such as:

• En la ventanilla de correos, la empleada me dijo que me faltaba otro sello.
• En correos, me dijeron que faltaba otro sello.
• La empleada de correos me dijo que necesitaba otro sello.

But your sentence is perfectly normal Spanish for Spain.