O plano mudou totalmente.

O is the masculine singular definite article, equivalent to the in English.

So:

• o plano = the plan

Portuguese nouns have grammatical gender, and plano is masculine, so it takes o.

Plano is a masculine singular noun in Portuguese. Because it is masculine singular, it normally goes with o:

• o plano = the plan
• um plano = a plan

A learner may notice that many masculine nouns end in -o, and plano follows that common pattern.

Mudou is the preterite form of the verb mudar.

Here it means that the change happened as a completed event in the past:

• mudar = to change
• mudou = changed

This is the 3rd person singular preterite form, used with subjects like:

• ele mudou = he changed
• ela mudou = she changed
• o plano mudou = the plan changed

In this sentence, mudou is a simple past form, so the most direct translation is changed.

However, depending on context, English might sometimes use:

• changed
• has changed

Portuguese preterite often corresponds to English simple past, but translation can vary a bit depending on the situation.

Portuguese often does not need a subject pronoun when the subject is already stated.

Here, the subject is explicitly given:

• O plano = the subject
• mudou = the verb

So there is no need to add anything like it before the verb. Portuguese normally just says:

• O plano mudou totalmente.

not something structured like English The plan it changed completely.

Here, mudar is being used intransitively.

That means the subject itself undergoes the change:

• O plano mudou. = The plan changed.

But mudar can also be transitive, where something changes something else:

• Mudaram o plano. = They changed the plan.

So this verb can work both ways, depending on the sentence.

Totalmente is an adverb meaning totally / completely, and in Portuguese adverbs often come after the verb.

So:

• mudou totalmente = changed completely

This word order is very natural in Portuguese. Putting totalmente at the end sounds normal and idiomatic.

Yes. Completamente is also very natural here.

Both of these work:

• O plano mudou totalmente.
• O plano mudou completamente.

They are very close in meaning. In everyday Brazilian Portuguese, both are common, though completamente can sometimes sound a little more neutral or standard, while totalmente can feel slightly more emphatic depending on tone and context.

Usually, the most natural version is:

• O plano mudou totalmente.

You may sometimes see adverbs in other positions for emphasis or style, but for a learner, it is safest to use totalmente after the verb here.

For example, O plano totalmente mudou would sound unnatural in normal usage.

In Brazilian Portuguese, mudou is pronounced approximately like:

• moo-DOH

More precisely:

• mu sounds like moo
• dou sounds roughly like doh, with the stress on this syllable

So the stress is on the last syllable:

• mu-DOU

A rough pronunciation guide for Brazilian Portuguese is:

• O plano mudou totalmente
• oo PLAH-noo moo-DOH toh-tahl-MEN-chee

A few notes:

• O often sounds like oo
• plano has stress on pla
• mudou has stress on the last syllable
• totalmente has stress on men
• In Brazilian Portuguese, te at the end often sounds like chee

These are different past tenses.

• mudou = changed / did change
  This is the preterite, used for a completed change.
• mudava = was changing / used to change
  This is the imperfect, used for ongoing, habitual, or background past actions.

So:

• O plano mudou totalmente. = the plan changed completely
• O plano mudava totalmente... would suggest a repeated or ongoing kind of change, which is not the idea here

Yes. Plano can have several meanings depending on context, such as:

• plan
• flat
• level
• plane in some technical contexts

But in this sentence, with mudou totalmente, the natural meaning is plan.

Yes, potentially. Plano can refer to different kinds of plans depending on context:

• a strategy or idea
• a project plan
• a phone plan
• a health plan
• sometimes even a layout or plan drawing

By itself, O plano mudou totalmente simply says the plan changed completely. The exact type of plan depends on the broader context.

The structure is very straightforward and close to English:

• O plano = the plan
• mudou = changed
• totalmente = completely

So it follows:

• subject + verb + adverb

That makes this sentence a very natural and simple Portuguese sentence pattern.