El plan nacional cambió.

Because plan is a masculine noun in Spanish, so it takes the masculine article el.

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

This is something you just have to learn with the noun. Even though nouns ending in -a are often feminine, plan does not end in -a, and it is masculine.

In Spanish, most adjectives normally come after the noun.

So:

• el plan nacional = the national plan

This is the most natural order in Spanish. Putting adjectives before the noun is possible in some cases, but it often changes the tone, emphasis, or style. With a straightforward descriptive adjective like nacional, the normal position is after the noun.

Nacional is an adjective, and it means national. It does agree in number, but not always in gender with a different form.

Here is how it works:

• masculine singular: nacional
• feminine singular: nacional
• masculine plural: nacionales
• feminine plural: nacionales

So both el plan nacional and la estrategia nacional use nacional in the singular. Many Spanish adjectives have the same form for masculine and feminine.

Because cambió with an accent is the third-person singular preterite form of cambiar:

• yo cambié = I changed
• tú cambiaste = you changed
• él/ella/usted cambió = he/she/it/you changed

So El plan nacional cambió means The national plan changed.

By contrast, cambio without an accent is usually:

• I change / I am changing (from the verb cambiar in the present: yo cambio), or
• a noun meaning change

The accent mark is important because it tells you this is a past-tense verb form.

The accent mark shows the stress and also helps distinguish this form from others.

• cambio = stress on cam-
• cambió = stress on the final -ó

This matters because cambió is the completed past form: changed.

In pronunciation, it sounds roughly like cam-byOH.

Cambió is in the preterite (pretérito indefinido in Spain).

The preterite is used for:

• completed actions in the past
• events seen as a whole
• a change that happened

So El plan nacional cambió presents the change as a completed event: the plan changed.

If you used the imperfect instead:

• El plan nacional cambiaba

that would sound more like was changing / used to change, which is a different idea.

Yes. Spanish often does not use subject pronouns when the subject is already clear.

In this sentence, the subject is explicitly stated as El plan nacional, so there is no need for a separate word like it.

Spanish commonly leaves out subject pronouns such as yo, tú, él, ella, or ello when they are unnecessary.

No, that would not be normal Spanish.

The natural order is:

• El plan nacional cambió.

In Spanish, noun + adjective is usually the default order. So plan nacional is correct, while nacional plan is not.

Because Spanish usually expresses tense inside the verb ending itself, rather than using an extra helping verb like English sometimes does.

English:

• changed
• did change

Spanish:

• cambió

That single verb form already carries the idea of past completed action.

A rough pronunciation would be:

el plan na-thyo-NAL kam-byOH

More specifically in much of Spain:

• el = el
• plan = plan
• nacional = na-thyo-NAL
  
  • the c before i is often pronounced like th in Spain
• cambió = kam-byOH

If you are using a Latin American pronunciation, nacional would sound more like na-syo-NAL instead.

Normally, no. In this sentence, el is needed because you are talking about the national plan, a specific thing.

• El plan nacional cambió. = The national plan changed.

If you remove the article and say Plan nacional cambió, it sounds incomplete or unnatural in standard Spanish unless it appears in a headline or very reduced note-taking style.

You would change the noun, the article, and the adjective to plural, and the verb would also become plural:

• Los planes nacionales cambiaron. = The national plans changed.

Breakdown:

• el → los
• plan → planes
• nacional → nacionales
• cambió → cambiaron

This shows agreement across the sentence.