Cambiaron el sistema ayer.

Cambiaron is the preterite (pretérito indefinido in Spain), the tense used for a completed action in the past.

It is the third-person plural form of cambiar:

• yo cambié
• tú cambiaste
• él/ella cambió
• ellos/ellas cambiaron

So here it means that the action was completed in the past.

Spanish often leaves out the subject pronoun because the verb ending already gives that information.

So cambiaron could mean:

• they changed
• you all changed (formal plural, ustedes)

In many real contexts, it can also work like an unspecified they in English, meaning something like someone / people / they changed it, without saying exactly who.

Because Spanish is a pro-drop language. That means subject pronouns such as yo, tú, ellos, etc. are often omitted.

The ending -aron already tells you the subject is third-person plural, so ellos is not necessary.

• Cambiaron el sistema ayer.
• Ellos cambiaron el sistema ayer.

Both are grammatical, but the version without ellos is more natural unless you want to emphasize they.

Spanish usually uses an article where English might not.

So el sistema is the normal way to say the system. Leaving out the article would usually sound incomplete or unnatural here.

This sentence is referring to a specific system, so el is expected.

Because sistema is a masculine noun.

A useful pattern is that many nouns ending in -ma and coming from Greek are masculine in Spanish, for example:

• el problema
• el programa
• el tema
• el sistema

So even though it ends in -a, it still takes el.

The preterite is used because the sentence describes a completed event: the change happened yesterday.

Ayer points to a specific finished time in the past, which fits the preterite very well.

Compare:

• Cambiaron el sistema ayer. = completed action yesterday
• Cambiaban el sistema... = they were changing / used to change the system

The imperfect would suggest an ongoing, repeated, or background action, which is not the idea here.

In standard Spanish from Spain, ayer normally goes with the preterite, not the present perfect.

So:

• Cambiaron el sistema ayer. = natural
• Han cambiado el sistema ayer. = generally not standard

In Spain, the present perfect is more often used with time expressions connected to the present, such as:

• hoy
• esta semana
• este mes

Since ayer is clearly a finished past time, cambiaron is the expected form.

Ayer means yesterday and gives the time of the action.

It helps show that the event happened at a specific point in the past, which is another reason the preterite is used.

It is a simple time expression and does not need a preposition here.

Yes, Spanish word order is fairly flexible.

Cambiaron el sistema ayer is a very natural, neutral order, but you could also say:

• Ayer cambiaron el sistema.
• Cambiaron ayer el sistema. (possible, though less neutral in many contexts)

Putting ayer at the end is common because it adds the time information after the main action.

No. This sentence means that someone/they changed the system.

Here, el sistema is the direct object of cambiaron.

If you want to say the system changed yesterday, you would say:

• El sistema cambió ayer.

That version makes el sistema the subject.

So compare:

• Cambiaron el sistema ayer. = they changed the system yesterday
• El sistema cambió ayer. = the system changed yesterday

Yes. In this sentence, cambiar is used transitively, meaning it takes a direct object: el sistema.

So the structure is:

• cambiaron = verb
• el sistema = thing that was changed
• ayer = time expression

The verb cambiar can also be used intransitively in other contexts, but here it clearly has an object.

It is pronounced roughly cam-BYA-ron.

The stress falls on bia because the word ends in -n, so Spanish stress rules place the stress on the penultimate syllable unless there is a written accent.

So:

• cam-bia-ron
• stress on bia

There is no written accent mark because the regular stress rules already produce the correct stress.