La policía advirtió al conductor.

In Spanish, la policía is often a collective singular noun meaning the police force. Because it is grammatically singular, the verb is singular too:

• La policía advirtió... = The police warned...

If you want to talk about individual officers, Spanish often uses the plural:

• Los policías advirtieron al conductor. = The police officers warned the driver.

So the Spanish grammar is singular here, even if the English translation may sound plural.

Yes. La policía can mean:

• the police / the police force
• the female police officer

So this phrase can be ambiguous without more context. If you specifically mean a male police officer, it would be el policía. If you mean police officers in the plural, it would be los policías.

Advirtió is the preterite form, third person singular, of advertir.

So it means that the warning happened as a completed action in the past:

• advirtió = warned

This is different from the imperfect:

• advertía = was warning / used to warn

So La policía advirtió al conductor presents the warning as a finished event.

Because advertir has a stem change in the preterite in the third person forms:

• advertí
• advertiste
• advirtió
• advertimos
• advertisteis
• advirtieron

Notice the change from e to i in:

• advirtió
• advirtieron

This is a normal pattern for many -ir verbs.

They show the correct stress.

• policía is stressed on cí: po-li-cí-a
  The accent also helps show that í-a is pronounced in separate syllables.
• advirtió is stressed on the last syllable: ad-vir-tió

Without the accent marks, a reader might place the stress incorrectly.

Because a + el contracts to al.

So:

• a el conductor → al conductor

This contraction is required.

A useful comparison:

• Voy al centro.
• Hablo al conductor.

But note that this only happens with the article el, not with the pronoun él:

• a él = never al

This is the personal a.

In Spanish, when a specific person is the direct object, you usually put a before that person:

• La policía advirtió al conductor.

Here, the driver is a specific human being, so Spanish uses a.

Compare:

• Veo al conductor. = I see the driver.
• Veo el coche. = I see the car.

People often get the personal a; things usually do not.

Because Spanish marks the object with the personal a.

• La policía = subject, the one doing the action
• al conductor = object, the person receiving the warning

So this means:

• The police warned the driver

not

• The driver warned the police

Even if the order changes, the grammar can still show the roles clearly:

• Al conductor lo advirtió la policía.

This still means The police warned the driver.

In Spanish, common nouns usually need a determiner such as el, un, este, etc.

So you would normally say:

• al conductor = to the driver
• a un conductor = to a driver

Just a conductor by itself is generally not natural here.

Also, el shows that it is a specific driver, not just any driver.

In standard Spanish, you would normally use lo:

• La policía lo advirtió. = The police warned him.

That is because al conductor is treated as a direct object here.

However, in much of Spain you may also hear le with a masculine singular person:

• La policía le advirtió.

That is due to leísmo, which is common in Spain. For learners, lo is the safest standard form to learn first, but le is something you will definitely hear in Spain.

Yes. In Spain, conductor is a normal and common word for driver.

It is often more general or formal than some alternatives. For example:

• conductor = driver
• chofer = chauffeur/driver, but less central in Spain than in some Latin American varieties

So el conductor is a very natural choice in a sentence like this.