Llame en caso de que se retrase el paquete.

Llame here is the usted command form of llamar.

• llama = tú command or he/she calls
• llamar = infinitive, to call
• llame = formal command addressed to usted

So the sentence is speaking formally to one person: Call ...

In form, llame can be both:

• the usted command
• the present subjunctive form of llamar

In this sentence, it functions as a command: Call ...

This is very common in Spanish: the usted and ustedes commands use subjunctive forms.

Spanish often leaves subject pronouns out because the verb ending already gives the information.

So instead of saying:

• Usted llame en caso de que...

Spanish normally just says:

• Llame en caso de que...

The formal tone is still clear from llame.

En caso de que means in case and introduces a possibility or precaution.

• en caso de que = in case
• si = if

They are similar, but not always interchangeable.

In this sentence, en caso de que sounds more like:

• Call as a precaution if the package ends up being delayed.

It is a very natural way to express in case in Spanish.

Because en caso de que normally triggers the subjunctive.

So after en caso de que, Spanish uses:

• se retrase = present subjunctive

not

• se retrasa = present indicative

This is because the delay is not presented as a fact. It is only a possible situation.

It comes from the verb retrasarse, which means to be delayed or to get delayed.

So:

• el paquete se retrasa = the package is delayed / gets delayed
• que se retrase el paquete = that the package be delayed / that the package gets delayed

The se is part of the verb retrasarse here. It is not a direct object pronoun.

In this sentence, retrasarse is being used in a way that means the package itself experiences a delay.

Both are possible.

• en caso de que se retrase el paquete
• en caso de que el paquete se retrase

Both mean the same thing.

The version with the verb before the noun can sound a little more natural or fluid in some contexts, especially in formal instructions. Spanish word order is often more flexible than English word order.

Usually, no, not with this meaning.

• retrasarse = to be delayed
• retrasar algo = to delay something

Compare:

• El paquete se retrasa. = The package is delayed.
• La empresa retrasa el paquete. = The company delays the package.

In your sentence, the package is the thing experiencing the delay, so retrasarse is the natural choice.

Yes, llame is formal, for usted.

Informal tú version:

• Llama en caso de que se retrase el paquete.

Plural versions:

• Llamen... = informal plural in most of Latin America, or formal plural everywhere
• Llámame... would mean Call me..., which changes the sentence

So if you want the same sentence but informal singular, use llama.

Llame does not need an accent because its natural stress already follows normal Spanish stress rules.

But when pronouns are attached, stress can shift:

• Llame = no accent needed
• Llámeme = call me; the written accent keeps the stress in the correct place

So the accent is not because it is a different tense. It is there to preserve pronunciation when extra pronouns are added.

Yes, that is possible and understandable.

• Llame si se retrasa el paquete. = Call if the package is delayed.
• Llame en caso de que se retrase el paquete. = Call in case the package is delayed.

The version with en caso de que sounds a bit more formal and more explicitly precautionary. The version with si is simpler and more direct.

Yes. It sounds quite natural in formal written Spanish, especially in:

• customer service messages
• shipping notices
• business communication
• instructions to clients

The use of:

• llame
• en caso de que
• se retrase

gives it a polite, formal, and somewhat official tone.

By itself, llame only means call. It does not specify whom to call.

So the sentence leaves the object understood from context. In real life, the surrounding situation would tell you whether it means:

• call the company
• call customer service
• call the sender
• call us

If Spanish wanted to make that explicit, it could say something like:

• Llámenos en caso de que se retrase el paquete. = Call us if the package is delayed.