Essa ação mudou tudo.

Because ação is a feminine noun in Portuguese.

• o / esse / um go with masculine nouns
• a / essa / uma go with feminine nouns

So:

• essa ação = correct
• esse ação = incorrect

A good thing to memorize is the noun together with its article:

• a ação

That makes it easier to remember that it is feminine.

Essa means that in this sentence.

It is the feminine singular form of the demonstrative:

• esse / essa = that
• este / esta = often this
• aquele / aquela = that over there / that one further away

In everyday Brazilian Portuguese, though, esse/essa is very commonly used where English speakers might expect either this or that, depending on context. So in real life, the distinction is often less strict than in grammar books.

Because ação is feminine, the form has to be essa.

The word ação has a ç and ã, and both matter.

1. ç

The ç is called cedilha. It makes the c sound like s before a, o, or u.

So:

• ação sounds like assão, not akão

2. ã

The ã shows a nasal vowel. You do not pronounce it like a plain a. Air comes through both the mouth and the nose.

So ação is roughly pronounced like:

• ah-SOWN
  or more carefully:
• a-SÃW̃

The final -ão is a very common Portuguese ending and is one of the sounds English speakers often need extra practice with.

Mudou is pronounced roughly like moo-DOH, with the stress on the second syllable.

Breakdown:

• mu = like moo
• dou = like doh with a glide at the end

In IPA, it is approximately /muˈdow/.

A common mistake is to pronounce every vowel separately, like mu-do-u. Do not do that. The ending -ou forms one syllable here.

Mudou is the simple past in Portuguese, also called the pretérito perfeito.

It comes from the verb mudar = to change.

Conjugation:

• eu mudei = I changed
• você/ele/ela mudou = you/he/she changed
• nós mudamos = we changed
• vocês/eles/elas mudaram = you/they changed

So Essa ação mudou tudo means that the action changed everything as a completed event.

Because here mudar is being used as a regular transitive verb meaning to change something.

Structure:

• Essa ação = subject
• mudou = changed
• tudo = everything

So the sentence means:

• That action changed everything

If you used se mudou, that would suggest a reflexive or different structure, often related to moving or changing oneself depending on context, and it would not match this sentence.

Compare:

• Essa ação mudou tudo. = That action changed everything.
• Ela se mudou. = She moved house / She relocated.

So mudou without se is exactly right here.

Because tudo and todo are not the same thing.

tudo

Means everything or all of it.

• mudou tudo = changed everything

todo / toda / todos / todas

Usually means all, whole, or every, and it normally goes with a noun.

Examples:

• todo dia = every day
• toda a cidade = the whole city
• todos os livros = all the books

So in this sentence:

• mudou tudo = correct
• mudou todo = incorrect by itself

Yes, you can, and it is grammatically correct.

Difference:

• esta ação = this action
• essa ação = that action

However, in spoken Brazilian Portuguese, people often use esse/essa very broadly, even where older textbook rules might prefer este/esta.

So:

• Esta ação mudou tudo. = correct, a bit more formal or careful
• Essa ação mudou tudo. = very natural in Brazilian Portuguese

In many everyday situations, Brazilians would prefer essa.

Because essa already works as a determiner before the noun.

In Portuguese, demonstratives like esse, essa, este, esta, aquele, aquela usually come directly before the noun:

• essa ação
• este livro
• aquela casa

You do not say:

• a essa ação in this sentence

You can have an article plus demonstrative in some contexts, but then the structure and meaning are different. For example:

• Referi-me a essa ação. = I referred to that action.

Here, a is a preposition, not just an article.

In Essa ação mudou tudo, the simple noun phrase essa ação is exactly what you want.

The most natural order is:

• Essa ação mudou tudo.

That is the standard subject + verb + object order.

You can change word order in Portuguese sometimes, but Mudou tudo essa ação sounds unnatural in normal speech unless there is a very specific poetic or emphatic context.

Other possible versions with different emphasis might be:

• Tudo mudou com essa ação. = Everything changed because of / with that action.
• Foi essa ação que mudou tudo. = It was that action that changed everything.

So yes, Portuguese allows flexibility, but the original sentence is the normal and natural version.

Not always. Ação can have several meanings depending on context, such as:

• action / act
• initiative / measure
• stock/share in finance
• action in the sense of movie genre
• sometimes lawsuit/legal action

In Essa ação mudou tudo, the most likely meaning is something like:

• That action
• That act
• That move
• That step

So even if the direct translation is action, the natural English wording may vary depending on context.

A Brazilian speaker might stress different words depending on what they want to highlight.

Neutral:

• Essa ação mudou tudo.

Emphasizing the action:

• Essa ação mudou tudo.

Emphasizing the result:

• Essa ação mudou tudo.

Stronger emphasis:

• Foi essa ação que mudou tudo. = It was that action that changed everything.

That last version is especially common when the speaker wants to be very clear about the cause.

Yes, depending on context, mudou tudo can be understood a bit more broadly.

Literally, it means:

• changed everything

But in natural use, it can also suggest:

• changed things completely
• changed the whole situation
• changed it all
• changed everything about it

So the phrase can be both literal and slightly idiomatic, depending on what is being discussed.