Si la destinataria no estaba en casa, el remitente tenía que volver a la oficina de correos con el código de referencia.

Because this sentence is describing a situation or rule in the past, not a single completed event.

• estaba is the imperfect of estar. It describes a state: the recipient was not at home.
• tenía que is also imperfect. It expresses an ongoing obligation or a usual rule in the past: the sender had to / was supposed to return.

If you used estuvo or tuvo, the sentence would sound more like one specific completed event, rather than background information or a general procedure.

Because this sentence is talking about a real or possible situation in the past, not a hypothetical or unlikely one.

With si, Spanish often uses:

• si + imperfect indicative for real past situations
  
  • Si no estaba en casa...
• si + imperfect subjunctive for hypothetical or unreal situations
  
  • Si no estuviera en casa... = If she were not at home...

So here, si ... estaba means something like if the recipient was not at home in an actual situation.

They refer to opposite roles in sending mail:

• la destinataria = the female recipient, the person receiving the letter or parcel
• el remitente = the sender, the person who sent it

A useful thing to notice is that:

• destinataria changes for gender:
  
  • destinatario = male recipient
  • destinataria = female recipient
• remitente usually has the same form for masculine and feminine, but the article changes:
  
  • el remitente
  • la remitente

Yes, in this sentence el remitente suggests a male sender, while la destinataria suggests a female recipient.

Spanish often marks gender through the article:

• la destinataria = female recipient
• el remitente = male sender

If the sender were female, it would normally be:

• la remitente

So the sentence is gender-specific as written.

Because en casa means at home, while a casa means homeward / to home after a verb of movement.

Compare:

• estaba en casa = she was at home
• volvió a casa = she returned home

In your sentence, the idea is location, not movement, so en casa is correct.

Because volver means to return, and when you say where someone returns to, Spanish uses a.

So:

• volver a la oficina de correos = to return to the post office

This is a very common pattern:

• volver a casa
• volver al trabajo
• volver a Madrid

It can suggest either, depending on context.

• tenía que + infinitive literally means had to
• But in many contexts, especially when describing rules or procedures, it can also mean was supposed to or was required to

So here it probably means there was a rule or obligation: if the recipient was not at home, the sender had to / was required to go back to the post office.

La oficina de correos means the post office as a physical place.

• oficina = office
• de correos = of post/mail

In Spain, Correos is also the name of the national postal service, so depending on context you may also hear things like:

• ir a Correos = to go to the post office / to the postal service office

But la oficina de correos is a clear, standard way to refer to the post office building.

It means with the reference code.

• con = with
• el código = the code
• de referencia = reference / identifying

This kind of phrase is very common in administrative or postal Spanish. A código de referencia is a code used to identify a package, form, delivery, or transaction.

In this sentence, el código de referencia refers to a specific code already known in the situation, so Spanish uses the definite article.

• con el código de referencia = with the reference code

If you said con código de referencia, it would sound more general or incomplete, like with reference code rather than with the reference code.

Spanish uses definite articles more often than English does, especially when referring to something specific in context.

Yes, absolutely. That would still be correct.

These two versions are both fine:

• Si la destinataria no estaba en casa, el remitente tenía que volver...
• El remitente tenía que volver... si la destinataria no estaba en casa.

Starting with the si clause puts the condition first, which often sounds natural when explaining a rule or procedure.

Also notice the comma:

• When the si clause comes first, a comma is normally used.
• When it comes second, the comma is usually omitted.

It is common in formal, postal, legal, or administrative contexts, but less common in everyday conversation.

In ordinary speech, people might more often say things like:

• la persona que lo recibe = the person who receives it
• la receptora in some contexts, though less postal-specific

But for letters, parcels, documents, and official notices, destinataria is a very normal and useful word.

Sometimes in practical instructions, si can feel close to when, especially if the speaker expects the situation to happen. But grammatically it still means if.

So:

• Si la destinataria no estaba en casa... = If the recipient was not at home...

In context, English might sometimes express the same idea more naturally as when the recipient was not at home, but the Spanish word used is still si, not cuando.

Because in Spanish, negation usually goes directly before the conjugated verb.

So:

• no estaba = was not
• no tenía = did not have
• no volvió = did not return

That is the normal Spanish word order for negation.