La policía persiguió al ladrón.

In Spanish, la policía often refers to the police force as an institution, and it is treated as a singular feminine noun.

So:

• La policía persiguió al ladrón = The police chased the thief
• Grammatically, la policía is singular, even though in English the police is usually plural.

If you want to talk about individual police officers, you could say:

• Los policías persiguieron al ladrón = The police officers chased the thief

So the Spanish sentence is using the institution/force idea, not emphasizing individual officers.

Because policía, when it means the police force, is a feminine noun in Spanish, so it takes la.

• la policía = the police force

Be careful: policía can also mean police officer, and then the article changes depending on the person:

• el policía = the male police officer
• la policía = the female police officer

In your sentence, la policía does not mean the female police officer. It means the police force.

Persiguió is the preterite tense, third person singular, of perseguir.

It means:

• he/she/it chased
• you (formal) chased

In this sentence, it means the police chased.

The preterite is used for a completed action in the past, so the sentence presents the chase as a finished event.

Because the verb is perseguir, and in some forms the stem changes from e to i.

This is a stem-changing verb:

• infinitive: perseguir
• present: persigo, persigues, etc.

In the preterite, only certain forms keep the change, including the third person singular and third person plural:

• persiguió = he/she/it chased
• persiguieron = they chased

So persiguió is the correct form, not perseguió.

Al is a contraction of:

• a + el = al

It appears here because of the personal a.

In Spanish, when the direct object is a specific person or a personified being, you usually put a before it. A thief is a person, so Spanish says:

• persiguió al ladrón

Literally, that is something like chased to the thief, but in natural English it is just chased the thief.

So:

• a = required because the object is a person
• el ladrón = the thief
• a + el = al

Yes, that would be incorrect in standard Spanish.

Because el ladrón is a person, Spanish normally requires the personal a:

• correct: La policía persiguió al ladrón
• incorrect: La policía persiguió el ladrón

This is a very common difference from English, because English does not add anything before the thief, but Spanish usually does.

The accent mark shows the correct stress and also helps distinguish the form.

• persiguió is stressed on the last syllable: per-si-guió

In writing, the accent is needed because this verb form ends in a vowel and would otherwise follow different default stress rules.

It also helps identify it as the third person singular preterite form.

Many Spanish past tense forms like this have written accents:

• habló
• comió
• vivió
• persiguió

Ladrón has an accent because the stress falls on the last syllable:

• la-drón

Words ending in n, s, or a vowel are normally stressed on the second-to-last syllable unless there is a written accent. Since this word is stressed on the last syllable, it needs the accent mark.

So the accent tells you how to pronounce it correctly.

Yes, Spanish word order is more flexible than English, though the original sentence is the most neutral and natural.

Standard order:

• La policía persiguió al ladrón.

You may also see other orders in certain contexts, for emphasis:

• Al ladrón lo persiguió la policía.
  This emphasizes the thief.
• Persiguió la policía al ladrón.
  Possible, but less neutral and more stylistic.

For a learner, the basic subject + verb + object order is the safest choice.

Normally, no. In this exact sentence, la policía is most naturally understood as the police force, not the female police officer.

If you wanted to say the police officer clearly, you would usually rely on context or use wording that makes it obvious:

• La agente de policía persiguió al ladrón.
• La policía persiguió al ladrón could theoretically mean the female police officer chased the thief, but without context, most people will read it as the police force.

So the sentence usually means the police chased the thief.

Yes, but it means something slightly different.

• La policía persiguió al ladrón = the police chased the thief
  
  • a completed past event
• La policía estaba persiguiendo al ladrón = the police was/were chasing the thief
  
  • an action that was in progress in the past

So the original sentence focuses on the event as a whole, while estaba persiguiendo emphasizes the action as ongoing.

Ladrón is the masculine form meaning male thief or a thief of unspecified gender in some contexts.

The feminine form is:

• ladrona = female thief

So you could also say:

• La policía persiguió a la ladrona. = The police chased the female thief.

Notice that the personal a is still there, and now it combines with la differently:

• a la ladrona stays a la ladrona
• only a + el contracts to al