El idiota cambió el plan.

Because idiota is one of those Spanish nouns that can refer to either a man or a woman, and the article shows the gender:

• el idiota = a male idiot
• la idiota = a female idiot

So the ending -a does not always mean the noun is feminine. Spanish has quite a few exceptions like this.

Cambió is the 3rd person singular preterite of cambiar.

That means it translates as:

• he changed
• she changed
• it changed
• or, in this sentence, the idiot changed

The preterite is used for a completed action in the past.

The accent mark is important because it shows both pronunciation and helps distinguish forms.

• cambio = I change / I am changing / change (noun, depending on context)
• cambió = he/she changed

So the accent tells you this is the past tense form, not the yo form or the noun.

Because plan is a masculine singular noun, and here it is being used as the plan.

So:

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

In Spanish, each noun normally needs its own article when the meaning is definite, so you say El idiota cambió el plan, not just El idiota cambió plan.

Yes. Noun gender in Spanish is not determined only by the ending.

• el plan = masculine
• plural: los planes

You simply have to learn many nouns together with their article.

This is the most neutral and straightforward Spanish word order:

• Subject: El idiota
• Verb: cambió
• Object: el plan

So it follows a normal subject–verb–object pattern, just like English.

Spanish can sometimes move things around for emphasis, but this version is the plain, natural order.

Yes, it could.

Spanish often omits subject pronouns and sometimes even leaves the subject unstated if the context makes it clear. So:

• Cambió el plan = He/She changed the plan

But if you want to make it explicit who did it, or emphasize the subject, you include El idiota.

Because el idiota is the subject, not the direct object.

The personal a is used before a specific human direct object, for example:

• Veo al idiota = I see the idiot

But in your sentence:

• El idiota = the person doing the action
• el plan = the thing receiving the action

So no personal a is needed.

It is pronounced roughly kam-BYOH.

A few helpful points:

• cam sounds like kam
• bió is one syllable, like byoh
• the stress falls on the last syllable because of the accent: cam-BIÓ

So the whole word is kam-BYOH.

Yes, idiota is definitely an insult. It means idiot and can sound harsh, depending on tone and context.

Like in English, it can sometimes be used jokingly among friends, but you should be careful with it because it can easily sound offensive.

Usually the most natural translation is The idiot changed the plan.

The verb cambiar most commonly means to change. In some contexts it can overlap with switch, but for this sentence changed is the clearest and most direct translation.

You would usually change the article:

• La idiota cambió el plan.

That means The female idiot changed the plan.

The verb cambió stays the same, because Spanish past-tense verbs do not change according to the gender of the subject.