Learn Hardcore Portuguese (Portugal): Ela foi muito corajosa quando decidiu mudar de profissão. - She was very brave when she decided to change profession.

Ela foi muito corajosa… = She was (at that moment / on that occasion) very brave.
→ A single, completed situation, seen as a specific event in the past.
Ela era muito corajosa… = She used to be / was generally very brave.
→ A habitual or ongoing characteristic in the past.

In this sentence, the focus is on one particular decision and the bravery at that point in time, so foi is the natural choice.

Why is it muito corajosa and not muita corajosa?

Because here muito is an adverb (“very”), not an adjective (“much / many”).

As an adverb, muito is invariable: it does not change for gender or number.
- muito corajosa (very brave – feminine)
- muito corajoso (very brave – masculine)
- muito felizes (very happy – plural)
As an adjective, muito / muita / muitos / muitas means much / a lot of / many and does change:
- muita coragem (a lot of courage)
- muitos amigos (many friends)

So, in muito corajosa, you are intensifying an adjective, so you must use muito (adverb), not muita.

Why does corajosa end in -a?

Because adjectives in Portuguese usually agree in gender and number with the noun or pronoun they describe.

The subject is ela (she), which is feminine singular.
The base adjective is corajoso (brave).
To match ela, you use the feminine singular form: corajosa.

Examples:

Ele é corajoso. – He is brave.
Ela é corajosa. – She is brave.
Eles são corajosos. – They are brave (group of men / mixed).
Elas são corajosas. – They are brave (group of women).

Could we leave out ela and just say Foi muito corajosa…?

Yes, you can omit ela. Portuguese is a “null subject” language, so subject pronouns are often dropped when the verb form already shows who the subject is.

Ela foi muito corajosa quando decidiu mudar de profissão.
Foi muito corajosa quando decidiu mudar de profissão.

Both are correct. Omitting ela is normal if it’s already clear from context who you are talking about. If you need to avoid ambiguity or emphasize “she (and not someone else)”, you keep ela.

Why is decidiu (preterite) used, and not something like decidia?

Decidiu is the pretérito perfeito (simple past), used for completed actions at a specific time.

quando decidiu mudar de profissão
→ “when she decided to change profession” – one concrete decision.

Decidia (imperfeito) would suggest an ongoing or repeated action in the past (“was deciding”, “used to decide”), which does not fit a single, clear decision.

So you have:

Ela foi muito corajosa (she was very brave – on that occasion)
quando decidiu mudar de profissão (when she made that decision)

Both verbs are in the same past tense (pretérito perfeito), matching the idea of a specific moment in the past.

Why do we say mudar de profissão and not mudar profissão or mudar a profissão?

With things like jobs, houses, clothes, etc., Portuguese often uses the structure mudar de + noun to mean “to change / switch to another X”.

mudar de profissão – to change profession
mudar de casa – to move house
mudar de roupa – to change clothes

Using mudar without de usually means you are changing something in some way, not replacing it with another:

mudar a profissão – to change the profession (to alter it somehow, not to have a different job yourself). This is unusual in this personal sense.

So for changing to a different job, you normally say mudar de profissão.

Why is there no article before profissão? Why not mudar de a profissão?

Because in Portuguese it’s very common to omit the article in certain fixed patterns, especially:

after the preposition de when you mean “change of X” in a general sense:
- mudar de profissão – change profession
- mudar de casa – move house
- mudar de escola – change school

Adding the article would sound odd or would usually give a different, more specific meaning:

mudar da profissão would suggest something like change from the profession (with a more concrete reference), and is not the standard way to say “change profession”.

So the natural, idiomatic form is mudar de profissão, with no article.

Could we say trocar de profissão instead of mudar de profissão? Is there any difference?

Yes, you can also say trocar de profissão.

mudar de profissão – to change profession
trocar de profissão – to switch profession

In many contexts, they are practically synonymous. Any nuance difference is small:

mudar de is slightly more neutral and more frequent.
trocar de literally emphasizes “exchange / swap one for another”, but in everyday speech both are fine for “change career”.

Your sentence would still be perfectly natural as:

Ela foi muito corajosa quando decidiu trocar de profissão.

Can we change the word order, like: Ela foi muito corajosa quando mudar de profissão decidiu?

No, that word order is not natural in Portuguese. The typical order is:

Ela foi muito corajosa quando decidiu mudar de profissão.
(Subject – verb – complement; then quando
- verb + infinitive.)

You can move the quando-clause to the beginning:

Quando decidiu mudar de profissão, ela foi muito corajosa.

But you cannot split mudar de profissão decidiu in that way. The normal pattern is:

decidiu + infinitive → decidiu mudar de profissão (“decided to change profession”)

What’s the difference between profissão, trabalho, and emprego here?

They are related but not identical:

profissão – profession / occupation / career path
- Often suggests a field with some training or qualification: profissão de médica, advogada, engenheira.
trabalho – work / job / labor (more general)
- Can mean the activity you do, your job, or “work” in the abstract:
  - Tenho muito trabalho. – I have a lot of work.
emprego – employment / (a) job / position
- A concrete job you are hired to do:
  - Perdeu o emprego. – He/she lost his/her job.

So:

mudar de profissão – to change your career/profession.
mudar de emprego – to change jobs (e.g., from one company to another).

In your sentence, profissão emphasizes a career change, not just a job change within the same field.