La presión cambió el plan.

Because presión is a feminine noun in Spanish, so it takes la in the singular.

A very useful pattern is that many nouns ending in -ción, -sión, or -ión are feminine:

• la presión
• la nación
• la televisión

So here, la presión is the subject of the sentence.

The accent mark in presión shows where the stress falls: on the last syllable, -sión.

Without the accent, Spanish spelling rules would suggest a different stress pattern. The written accent helps show the correct pronunciation.

It is pronounced roughly like:

• pre-syon

Also notice that the plural is presiones, and the written accent disappears there because the stress pattern changes naturally.

Cambió is the preterite form of cambiar, third person singular, meaning changed.

So:

• cambia = changes / is changing
• cambiaba = was changing / used to change
• cambió = changed

In La presión cambió el plan, the sentence describes a completed action in the past, so cambió is the natural choice.

The accent mark in cambió shows that the stress is on the final syllable: cam-bi-Ó.

This is also useful because it distinguishes it from cambio:

• cambio = I change or change as a noun
• cambió = he/she/it changed

So the accent is important both for pronunciation and meaning.

In this sentence, la presión is the subject, so the pressure is what caused the change.

The structure is:

• La presión = subject
• cambió = verb
• el plan = direct object

So literally, it is The pressure changed the plan.

Spanish often leaves out subject pronouns like ella or eso, but here the subject is clearly stated as la presión.

Because plan is a masculine noun, so it takes el.

• el plan = the plan
• un plan = a plan

Even though plan ends in -n, not -o, it is still masculine. Noun gender in Spanish is not always predictable from the ending, so it is best to learn nouns together with their article:

• el plan
• la presión

Yes. This sentence follows the very common subject + verb + object order:

• La presión = subject
• cambió = verb
• el plan = object

That said, Spanish can sometimes change word order for emphasis. For example, you might see:

• El plan lo cambió la presión.

That sounds more marked and emphatic. For a normal, neutral statement, La presión cambió el plan is the most straightforward order.

Yes, it could be either.

La presión can mean:

• literal pressure: physical force
• figurative pressure: social, political, emotional, or work-related pressure

So this sentence could mean something like:

• physical pressure altered a plan
• outside pressure forced a change in the plan

You understand which one is meant from context, not from grammar alone.

Because el plan is a direct object, but it is not a person.

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

• Veo a María.
• Ayudaron al profesor.

But with things, you normally do not use a:

• cambió el plan
• compró el coche
• vio la película

So La presión cambió el plan is correct without a.

In standard Peninsular Spanish, it would sound approximately like:

• la pre-SYON kam-BYOH el plan

A few pronunciation notes:

• presión is stressed on the last syllable.
• cambió is stressed on the last syllable.
• In much of Spain, c before i or e sounds like the th in thin, but here the c in cambió is before a, so it sounds like a normal k sound.
• ll does not appear here, so there is no special issue there.

Yes. In Spain, that is very natural, especially if the change is connected to the present or still relevant now.

Compare:

• La presión cambió el plan. = The pressure changed the plan.
  A completed past event.

• La presión ha cambiado el plan. = The pressure has changed the plan.
  Often used when the result matters now or the time period still feels current.

In many parts of Spain, the present perfect is used more often than in some Latin American varieties for recent or present-relevant past actions.

Yes. Cambio can also be a noun.

For example:

• un cambio = a change

So these are different:

• Yo cambio el plan. = I change the plan.
• Hubo un cambio de plan. = There was a change of plan.
• La presión cambió el plan. = The pressure changed the plan.

That is another reason the accent in cambió matters so much.