Abandonaron la casa ayer.

Spanish often leaves out subject pronouns when the verb already shows who the subject is.
So abandonaron already tells you the subject is they or you all.

You could say Ellos abandonaron la casa ayer, but ellos is usually only added for emphasis, contrast, or clarity.

Abandonaron is the preterite form of abandonar.

It breaks down like this:

• abandonar = to abandon
• abandonaron = they abandoned / you all abandoned

The ending -aron is the regular preterite ending for -ar verbs in the third person plural.

Yes. Grammatically, abandonaron can mean either:

• they abandoned
• you all abandoned

In Spain, though, you all informally is usually vosotros, so the expected verb would be abandonasteis. Because of that, without more context, most learners should read abandonaron here as they abandoned.

It could still mean you all abandoned if the speaker is using ustedes, which is formal in most of Spain.

The preterite is used for a completed action in the past.
Here, the action happened yesterday and is presented as a finished event, so the preterite fits naturally:

• abandonaron = they abandoned

If the sentence were describing an ongoing situation or background, Spanish might use the imperfect instead, but not here.

Not exactly.

Abandonar is usually stronger than dejar. It often suggests:

• leaving something behind
• deserting it
• giving it up completely

So Abandonaron la casa ayer sounds more like They abandoned the house yesterday, not just They left the house yesterday in the simple sense of going out.

If you only mean they went out of the house, Spanish would more naturally use something like salieron de la casa.

Spanish uses definite articles more often than English.
Here, la casa means the house and refers to a specific house already known in the context.

Saying just casa would usually not work in this sentence.
Spanish normally needs the article here because casa is a specific direct object.

Because la casa is a thing, not a person.

Spanish uses the personal a mainly before specific human direct objects, and sometimes pets or personified beings. For example:

• Vieron a María
• Busco a mi perro

But with things, you normally do not use that a:

• Abandonaron la casa
• Compraron el coche

Yes. Spanish word order is flexible, especially with time expressions.

All of these are possible:

• Abandonaron la casa ayer
• Ayer abandonaron la casa

The version with ayer at the end is very natural and neutral.
Putting ayer first gives it a little more emphasis, as in Yesterday, they abandoned the house.

Yes, that is grammatically correct.

But in normal Spanish, subject pronouns are often omitted, so Abandonaron la casa ayer is more natural unless you want to stress they specifically.

For example, you might include ellos if you are contrasting them with someone else:

• Nosotros no, pero ellos abandonaron la casa ayer

Because it follows the normal Spanish stress rules.

Abandonaron ends in -n, so the stress naturally falls on the second-to-last syllable:

a-ban-do-NA-ron

Since that stress is regular and predictable, no accent mark is needed.

Then the verb would change to abandonasteis:

• Abandonasteis la casa ayer = You all abandoned the house yesterday

This is the usual informal you all form in Spain.
That is another reason why abandonaron is more likely to be understood as they abandoned unless context suggests ustedes.