Avisaron al agente de asistencia en tierra.

Avisaron is the 3rd person plural preterite of avisar.

• avisar = to inform, notify, tell, warn
• avisaron = they informed/notified or someone informed/notified

The preterite is used because the sentence presents the action as a completed event.

Spanish often omits subject pronouns when the verb ending already shows who the subject is.

So avisaron already tells you the subject is they. But in many cases, this they is not meant literally as a specific group. It can also work like an impersonal they in English:

• Avisaron al agente... = They notified the agent...
• very naturally in English: The agent was notified / Someone notified the agent

So the sentence does not need an explicit ellos.

Because a + el = al in Spanish.

This is a mandatory contraction:

• a + el agente → al agente

So al here is just the combination of:

• a = the preposition
• el = the masculine singular article

This is because of the personal a.

In Spanish, when the direct object is a specific person (or sometimes a person-like being), it usually takes a:

• Vieron al piloto
• Llamaron al médico
• Avisaron al agente

So the a does not mean to here in the usual directional sense. It is marking a person as the object of the verb.

For most learners, the safest way to understand it here is as the person affected by the verb, introduced with the personal a.

With avisar, usage can vary a bit depending on region and structure, but in a sentence like this, learners will often treat al agente as the main object of avisaron.

A useful practical point:

• a person after avisar is very common
• the thing being reported is often added with de:
  • Avisaron al agente del retraso = They informed the agent about the delay

In your sentence, that extra information is simply not stated.

It describes the type of agent.

• asistencia en tierra = ground assistance
• in airport/aviation contexts, this often refers to ground handling or ground support

So agente de asistencia en tierra is something like:

• ground assistance agent
• ground handling agent
• ground support agent

Literally, en tierra means on the ground, as opposed to in the air / in flight.

Because noun + de + noun is a very common way in Spanish to show type, function, or area of work.

So:

• agente de asistencia en tierra
• centro de salud
• tarjeta de embarque

Here, de means something like for / connected with / in charge of, depending on context. Spanish prefers this structure very often where English may use an adjective or compound noun.

Yes. Agente is a noun that can refer to a male or female person.

But in this sentence, al agente contains el, so it is referring to a masculine singular agent.

If it were female, you would normally expect:

• Avisaron a la agente de asistencia en tierra.

So the word agente itself can be used for either sex, but the article helps show which one is meant.

Because the sentence presents the notification as a finished action.

• Avisaron... = they notified / they informed
• Avisaban... = they were notifying / they used to notify

So:

• Avisaron al agente... focuses on the completed event
• Avisaban al agente... would sound more like background, habit, or an action in progress in the past

Not necessarily. Avisar can appear without explicitly stating the message if the context already makes it clear.

For example:

• Avisaron al agente. = They notified the agent.
• Avisaron al agente del cambio de puerta. = They notified the agent about the gate change.

So your sentence is perfectly grammatical even though the thing communicated is left unstated.

Yes, Spanish word order is fairly flexible, though the version you have is very natural and neutral.

Possible alternatives include:

• Al agente de asistencia en tierra lo avisaron.
• In Spain, you may also hear le with a masculine person: Al agente de asistencia en tierra le avisaron.

But the basic, straightforward order is:

• Avisaron al agente de asistencia en tierra.

That is probably the most neutral version for a learner to start with.