La policía encontró el arma.

In Spanish, la policía is usually treated as a singular collective noun meaning the police force or the police service. So it takes a singular verb:

• La policía encontró el arma. = The police found the weapon.

If you want to talk about individual officers, you would use a plural noun instead:

• Los policías encontraron el arma. = The police officers found the weapon.

Yes, depending on context, la policía can also mean the female police officer. But in a sentence like La policía encontró el arma, the most natural reading is usually the police force rather than one woman officer.

Compare:

• La policía encontró el arma. = The police found the weapon.
• La policía habló con el testigo. could mean The policewoman spoke with the witness if the context is about one officer.

If you want to be very clear, Spanish often uses:

• la agente de policía = the policewoman / female police officer
• el policía = the policeman / male police officer
• los policías = the police officers

Great question. Arma is indeed a feminine noun, but feminine nouns that begin with a stressed a- or ha- sound usually take el in the singular for ease of pronunciation.

So you say:

• el arma
• el agua
• el águila

But the noun is still feminine. You can see that in other words that agree with it:

• el arma peligrosa = the dangerous weapon
• esta arma = this weapon

In the plural, the normal feminine article returns:

• las armas

Because only words that agree with the noun change. In this sentence, there is no adjective modifying arma, so you do not see much feminine agreement.

You only see the special article el before it. If you added an adjective, the feminine would become obvious:

• el arma pequeña
• el arma escondida

So arma is still feminine even though it uses el in the singular.

Encontró is the preterite form of encontrar, third person singular:

• yo encontré
• tú encontraste
• él/ella/usted encontró

It is used here because the sentence describes a completed action in the past: the police found the weapon.

So:

• La policía encontró el arma. = The police found the weapon.

Because encontró and encontraba express different kinds of past actions.

• encontró = found → a completed event
• encontraba = was finding / used to find / found in a descriptive or ongoing sense

In this sentence, the action is a single completed event, so encontró is the natural choice.

Compare:

• La policía encontró el arma ayer. = The police found the weapon yesterday.
• La policía buscaba pruebas cuando encontró el arma. = The police were looking for evidence when they found the weapon.

The accent mark shows where the stress goes and helps identify the verb form.

• encontró = he/she/it found, you found (formal), or the police found
• Stress is on the final syllable: en-con-TRÓ

Without the accent, encontro would not be the standard Spanish spelling for this form.

The personal a is used mainly before a specific person (and sometimes pets or personified beings) when they are a direct object.

Here, el arma is a thing, not a person, so there is no personal a:

• La policía encontró el arma.

Compare with a person:

• La policía encontró a un sospechoso. = The police found a suspect.

Arma usually means weapon in a general sense. Depending on context, English may translate it as weapon, and sometimes more specifically as gun, but the Spanish word itself is broader.

Examples:

• arma = weapon
• arma de fuego = firearm
• pistola = pistol
• revólver = revolver

So in isolation, el arma is usually best understood as the weapon.

In standard Spain Spanish, approximately:

la po-li-THÍ-a en-kon-TRÓ el AR-ma

A few pronunciation notes:

• policía has the stress on -cí-
• In most of Spain, c before i is pronounced like th in think, so policía sounds like poli-THÍ-a
• encontró is stressed on the last syllable
• h is silent in Spanish, but there is no h here

Yes, but La policía encontró el arma is the most neutral, straightforward order.

Spanish can change word order for emphasis:

• El arma la encontró la policía. = It was the police who found the weapon.
• Encontró el arma la policía. = less neutral, more marked or literary

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