La policía encerró al ladrón.

Because la policía here means the police as an institution or police force, and that noun is grammatically feminine singular in Spanish.

• la policía = the police / police force
• el policía = a male police officer
• la policía can also mean a female police officer in some contexts, but in this sentence it most naturally means the police

So La policía encerró al ladrón means The police locked up the thief, not The female police officer locked up the thief unless the context clearly points to one woman.

Because in Spanish, la policía is treated as a singular collective noun in this meaning.

So Spanish says:

• La policía encerró... = literally The police force locked up...

Even though English uses a plural-looking idea (the police), Spanish grammar treats la policía as singular here, so the verb is singular too:

• encerró = he/she/it locked up

Encerró is the preterite (completed past) form of encerrar, which means:

• to lock up
• to shut in
• to confine

In this sentence, encerró means locked up or put in jail/cell, depending on context.

Form breakdown:

• encerrar = infinitive
• encerró = he/she/it locked up

Because the subject is la policía, the full sense is:

• La policía encerró... = The police locked up...

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

Here, the thief is:

• a person
• specific

So you get:

• a + el = al
• encerró al ladrón

Compare:

• Veo al hombre. = I see the man.
• Arrestaron al sospechoso. = They arrested the suspect.

This does not mean to the thief here. It is just the required personal a.

Yes. Al is the mandatory contraction of:

• a + el = al

So:

• a el ladrón ❌
• al ladrón ✅

But there is no contraction with la:

• a la ladrona = to / personal a before the female thief

Examples:

• Vio al niño.
• Llamó a la policía.

Because the stress falls on the last syllable: la-drón.

Spanish spelling rules say that words ending in -n, -s, or a vowel are normally stressed on the second-to-last syllable unless there is a written accent mark. So the accent in ladrón shows that the stress is irregular and must be on the end.

• ladrón = thief
• plural: ladrones

Notice that the written accent disappears in the plural because the stress pattern changes naturally:

• la-DRÓN
• la-DRO-nes

Yes, but the normal neutral order is:

• La policía encerró al ladrón.

Spanish is more flexible with word order than English, but changing it usually changes the emphasis.

For example:

• Al ladrón lo encerró la policía.
  This emphasizes the thief
• Encerró la policía al ladrón.
  Grammatically possible, but less neutral and more stylistic

For most learners, the original version is the best standard pattern:

• subject + verb + object

Spanish uses definite articles (el, la, los, las) more often than English.

Here:

• la policía = the police
• al ladrón = the thief

In English, sometimes articles are dropped in general expressions, but in Spanish they are often required.

Also, al ladrón contains the article el inside al:

• a + el ladrón = al ladrón

So both nouns are definite and specific in this sentence.

Because encerró is the preterite, which presents the action as a completed event.

• encerró = locked up / did lock up
• encerraba = was locking up / used to lock up

So:

• La policía encerró al ladrón. = The police locked up the thief.
  A completed action
• La policía encerraba al ladrón. = The police was locking up the thief / used to lock up the thief.
  Ongoing, repeated, or background action

In a simple one-off event, encerró is the natural choice.

A careful Spain pronunciation would be approximately:

• la po-li-THI-a en-se-RRÓ al la-DRÓN
  in much of Spain, where c before i sounds like th in thin

Notes:

• policía has the stress on -cí-
• encerró has the stress on the last syllable
• rr in encerró is a rolled or strong r
• ladrón is stressed on the end

In many parts of Latin America, policía would sound more like:

• po-li-SI-a

Both are correct in their own varieties, but for Spain Spanish, th in policía is typical.

Sometimes, but the meaning is not exactly the same.

• arrestó = arrested
• encerró = locked up / shut in / confined

So:

• La policía arrestó al ladrón. = The police arrested the thief.
• La policía encerró al ladrón. = The police locked up the thief.

Usually, arrestar focuses on the act of taking someone into custody, while encerrar focuses on putting them inside a place such as a cell, room, or prison. Depending on context, both might fit, but they are not identical.