En la secretaría me dijeron que faltaba una firma en el formulario y que debía volver mañana.

Here la secretaría does not mean the secretary as a person. It means the administrative office, the office desk, or the registrar’s/department office, depending on the context.

So En la secretaría means something like:

• At the office
• In the administration office
• At the front office / registry desk

If you wanted to say the secretary (the person), that would normally be la secretaria.

So:

• la secretaria = the female secretary
• la secretaría = the office / administrative department

The accent mark helps distinguish them:

• secretaria
• secretaría

Because en tells you where something happened.

Here the sentence is setting the scene: in/at the office, they told me...

So:

• En la secretaría = At/In the office
• A la secretaría would usually suggest movement toward the office, as in to the office

Compare:

• En la secretaría me dijeron... = At the office, they told me...
• Fui a la secretaría... = I went to the office...

So en is used because the speaker is talking about the place where the conversation happened.

Me is the indirect object pronoun, meaning to me.

So:

• dijeron = they said / they told
• me dijeron = they told me

Spanish often includes this pronoun even when English may not emphasize it.

A useful pattern is:

• me dijeron = they told me
• te dijeron = they told you
• le dijeron = they told him/her/you
• nos dijeron = they told us

So the sentence literally begins:

• At the office, they told me that...

Dijeron is third person plural preterite of decir: they said / they told.

Spanish often uses the plural like this when:

• more than one person was involved, or
• the exact person is not important, or
• it is a general they in the English sense

So me dijeron can mean:

• they told me
• I was told in a natural English translation

If the speaker wanted to refer to one specific person, they might say:

• me dijo = he/she told me

In this sentence, me dijeron sounds natural because the focus is not on exactly who said it, but on the information the speaker received.

This is a very common question. Here faltaba is the imperfect, and it describes a state or situation that existed at that time: a signature was missing.

So:

• faltaba una firma = a signature was missing
• faltó una firma would sound more like a specific event: a signature failed to appear / was absent at a particular moment

In administrative situations, Spanish often uses the imperfect to describe what was lacking or needed:

• Faltaba un documento
• Faltaba una copia
• Faltaba una firma

It presents the missing item as a condition of the form, not as a sudden completed action.

So faltaba fits the idea of the form was incomplete because a signature was missing.

Because the sentence is referring to a missing signature, not to one already specifically identified for the listener.

• una firma = a signature
• la firma = the signature

In many real situations, una firma sounds natural because the office is simply pointing out that some required signature was missing.

If you said faltaba la firma, it would sound more like:

• a specific known signature was missing
• perhaps the one signature everyone expected

Both are possible in context, but faltaba una firma is very natural when talking generally about a form that needed one more signature.

Because en el formulario means on the form or in the form.

Spanish often uses en where English uses on for documents, pages, forms, lists, etc.

So:

• una firma en el formulario = a signature on the form

If you said del formulario, that would mean of the form or from the form, which is a different relationship.

Compare:

• Había un error en el formulario = There was an error on the form
• El nombre del formulario = The name of the form

So en el formulario is the natural choice here.

Because Spanish often repeats que when connecting two subordinate clauses after a verb like decir.

So the structure is:

• me dijeron que faltaba una firma
• y que debía volver mañana

Literally:

• they told me that a signature was missing
• and that I had to come back tomorrow

In English, we often drop the second that:

• They told me that a signature was missing and I had to come back tomorrow.

Spanish more often keeps it, especially in careful or natural written style.

Could you omit the second que? Sometimes in conversation people might, but repeating it is clearer and very standard.

Because the sentence is reporting what someone told the speaker in the past.

• debo volver = I must come back / I have to come back now
• debía volver = I had to come back / I was supposed to come back
• debí volver usually means something more like I should have come back or I must have come back, depending on context, so it would not fit here

In reported speech after me dijeron, Spanish commonly shifts into the past:

• me dijeron que debía volver mañana

This means the obligation existed at that past moment, as part of what they told the speaker.

So debía is the natural past form here.

Yes. Tenía que volver would also be very natural.

Both can mean I had to come back:

• debía volver
• tenía que volver

There can be a slight nuance:

• debía volver can sound a bit more formal, administrative, or written
• tenía que volver often sounds a bit more everyday and conversational

Because the sentence is about an office or administrative setting, debía volver fits very well.

So:

• me dijeron que debía volver mañana = slightly more formal
• me dijeron que tenía que volver mañana = very natural too

It means to return / to come back.

In this context, volver mañana means:

• to come back tomorrow
• to return tomorrow

Spanish uses volver very often for this idea. It is one of the most common verbs for coming back somewhere.

Examples:

• Tengo que volver mañana = I have to come back tomorrow
• Volví a casa = I went back home / I returned home

You may also see regresar, but volver is extremely common and natural in Spain.

Because Spanish often leaves subject pronouns out when they are understood from the verb.

Here debía volver can mean:

• I had to come back in reported speech from the speaker’s point of view

The idea is:

• They told me that I had to come back tomorrow

Spanish does not need to say yo unless it wants emphasis.

Also, if the original office staff probably spoke directly to the person, they may have said something like:

• Debe volver mañana or Tiene que volver mañana

But when the speaker reports it afterward, it becomes:

• me dijeron que debía volver mañana

So the sentence is not missing anything; Spanish simply omits the subject pronoun because it is clear.

In this sentence, mañana clearly means tomorrow.

That is because it follows volver:

• debía volver mañana = I had to come back tomorrow

Yes, mañana can also mean morning, but usually the context makes it clear.

Compare:

• Vuelvo mañana = I’m coming back tomorrow
• Vuelvo por la mañana = I’m coming back in the morning

So here there is no real ambiguity: it means tomorrow.

The sentence breaks down like this:

• En la secretaría = At the office
• me dijeron = they told me
• que faltaba una firma en el formulario = that a signature was missing on the form
• y que debía volver mañana = and that I had to come back tomorrow

So the core pattern is:

• [place] + me dijeron + que... + y que...

This is a very useful structure for reported speech in Spanish.

You can build similar sentences like:

• En recepción me dijeron que el médico llegaba tarde y que debía esperar.
• En la tienda me dijeron que no quedaba mi talla y que volviera la semana que viene.

It is a common and very practical pattern to learn.