Encerramos al perro en la casa.

Because Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

Encerramos already tells you the subject is we. So (nosotros) encerramos is optional.

You could add nosotros if you want emphasis or contrast:

• Nosotros encerramos al perro en la casa, no ellos.
• We locked the dog in the house, not them.

Without nosotros, the sentence still sounds completely natural.

It can mean either one, depending on context.

For -ar verbs, the nosotros form is the same in:

• the present: we lock up / we shut in
• the preterite: we locked up / we shut in

So encerramos can mean:

• we lock up
• we locked up

Context tells you which one is intended:

• Cada noche encerramos al perro en la casa. = Every night we lock the dog in the house.
• Ayer encerramos al perro en la casa. = Yesterday we locked the dog in the house.

Encerrar means to shut in, to lock in, to confine, or to shut up in a place.

So Encerramos al perro en la casa suggests that the dog was put inside the house and prevented from leaving.

It is stronger than just put. It carries the idea of confinement.

A useful comparison:

• cerrar = to close
• encerrar = to shut/lock someone or something inside

For example:

• Cerramos la puerta. = We closed the door.
• Encerramos al perro. = We shut/locked the dog in.

Because Spanish uses the personal a before a specific direct object that is a person, and often also with animals, especially pets or known animals.

Here, the dog is a specific, identifiable animal, so a is used:

• Encerramos al perro

This does not mean to the dog here. The dog is still the direct object.

Also, al is just the contraction of:

• a + el = al

So:

• a el perro is wrong
• al perro is correct

A helpful comparison:

• Busco un perro. = I’m looking for a dog (any dog)
• Busco a mi perro. = I’m looking for my dog (a specific dog)

No. In this sentence, al perro is the direct object.

That can be confusing for English speakers because a often looks like to, but here it is the personal a, not an indirect-object marker.

You can test it by asking What did we shut in? Answer: the dog.

So:

• Subject: implied we
• Verb: encerramos
• Direct object: al perro
• Place/complement: en la casa

Because en marks the place where the dog is confined: in the house.

With encerrar, Spanish normally uses en to show the place someone or something is shut inside:

• encerrar a alguien en un cuarto
• to lock someone in a room

Using a la casa would sound wrong with encerrar in this structure.

So:

• Encerramos al perro en la casa = We shut the dog in the house

If you wanted to focus on moving the dog into the house, a different verb might be more natural:

• Metimos al perro en la casa. = We put/brought the dog into the house.

Because en la casa refers to the house, a specific house.

Spanish often uses articles more than English does, and there is also a difference between:

• en casa = at home
• en la casa = in the house / in the home building

So these are not always the same:

• Estoy en casa. = I’m at home.
• Estoy en la casa. = I’m in the house.

In your sentence, en la casa points to the actual place where the dog was shut in.

Yes, Spanish word order is flexible, although the original sentence is the most neutral and straightforward.

Neutral order:

• Encerramos al perro en la casa.

You might also hear:

• Al perro lo encerramos en la casa.

That version puts extra focus on the dog, and the lo is a direct-object pronoun that repeats it.

Spanish often changes word order for emphasis, contrast, or style, but for a learner, the original order is the safest default.

In standard Spanish, that sounds wrong here.

Because the object is a specific animal, especially one understood as an individual or pet, standard Spanish uses the personal a:

• Encerramos al perro en la casa.

Leaving it out would sound ungrammatical or at least very unnatural in standard usage.

A good rule for learners is:

• use a with specific people
• also often use a with specific animals, especially pets or familiar animals

Yes. Dentro de la casa is possible and adds a bit more emphasis to the idea of being inside the house.

Compare:

• en la casa = in the house
• dentro de la casa = inside the house

So:

• Encerramos al perro dentro de la casa is also correct.

The original sentence is simpler and very natural.