Arrestaron al conductor ayer.

In Spanish, the 3rd person plural is often used when the subject is unknown, unimportant, or not stated.

So Arrestaron al conductor ayer literally means They arrested the driver yesterday, but in natural English it often becomes The driver was arrested yesterday.

This use is very common when the action matters more than who did it.

Examples:

• Llamaron a la puerta. = Someone/They knocked on the door.
• Me robaron el móvil. = Someone stole my phone.

Because arrestó is he/she/you-formal arrested, while arrestaron is they/you-plural arrested.

Compare:

• Arrestó al conductor ayer. = He/She arrested the driver yesterday.
• Arrestaron al conductor ayer. = They arrested the driver yesterday / The driver was arrested yesterday.

If you do not name the subject and you want the idea of an unspecified they, you use arrestaron.

It is the pretérito indefinido (also called the preterite or simple past).

It is used for a completed action in the past, especially with finished time expressions like ayer (yesterday).

So:

• arrestaron = they arrested

This tense is very natural here because the event happened at a specific finished time: ayer.

Because Spanish uses the personal a before a specific human direct object.

Here, the driver is a person and is the direct object of arrestaron, so Spanish requires a:

• Arrestaron a el conductor → this contracts to Arrestaron al conductor

So al here is:

• a = personal a
• el = the

And together:

• a + el = al

No. In this sentence, al conductor is the direct object, not the indirect object.

That can be confusing because English does not use the personal a, and Spanish also uses a with indirect objects.

A good test is the pronoun:

• Arrestaron al conductor.
• Lo arrestaron. = They arrested him.

Since lo is a direct object pronoun, that shows al conductor is the direct object.

Spanish usually omits subject pronouns when the verb already shows the person and number.

So instead of saying:

• Ellos arrestaron al conductor ayer

Spanish normally just says:

• Arrestaron al conductor ayer

Adding ellos is possible, but it usually gives extra emphasis or contrast.

Yes, Spanish word order is fairly flexible.

All of these are possible:

• Arrestaron al conductor ayer.
• Ayer arrestaron al conductor.
• Al conductor lo arrestaron ayer. (more marked/emphatic structure)

Putting ayer at the end is a very neutral, natural order.
Putting ayer first gives more emphasis to the time.

Yes. Even though the Spanish sentence is active in form, it often corresponds to a passive idea in English.

So:

• Arrestaron al conductor ayer.

can naturally be understood as:

• They arrested the driver yesterday.
• The driver was arrested yesterday.

Spanish often prefers this kind of active verb with an unspecified they instead of a passive structure.

Yes, that is also correct.

Compare the two:

• Arrestaron al conductor ayer. = active form with unspecified subject
• El conductor fue arrestado ayer. = true passive

Both are grammatical, but the first one is often more natural and common in everyday Spanish. The passive with fue arrestado can sound a bit more formal or journalistic.

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

It can refer to:

• a car driver
• a bus driver
• a truck driver

In some other Spanish-speaking regions, you may also hear words like chofer, but conductor is perfectly standard and widely understood.

In a neutral sentence, normally no.

If you already say al conductor, you usually do not also add lo unless the structure is doing something special, such as emphasis or dislocation.

Neutral:

• Arrestaron al conductor ayer.

More marked/emphatic:

• Al conductor lo arrestaron ayer.

So for a basic sentence, Arrestaron al conductor ayer is the best choice.

Arrestar is correct and understandable, but in Spain you will also very often hear detener.

For example:

• Detuvieron al conductor ayer. = They arrested/detained the driver yesterday.

In many contexts, detener sounds especially common in news and police-related language in Spain. But arrestaron is still perfectly valid Spanish.