Learn Hardcore Portuguese (Portugal): O irmão dela quer trabalhar em tecnologia médica, mas gosta muito de ciência pura. - Her brother wants to work in medical technology, but he really likes pure science.

The article o is there because we are talking about a specific, known brother.
Without the article (irmão dela) you can still be understood, but it sounds more telegraphic or less natural in normal speech.

Why is it dela and not de ela?

Dela is the contracted form of de + ela.

Portuguese contracts de with third-person pronouns:

de + ele → dele (of him / his)
de + ela → dela (of her / her)
de + eles → deles (of them / their – masculine or mixed)
de + elas → delas (of them / their – feminine)

So o irmão dela literally is o irmão de ela, but in correct Portuguese it must be contracted to dela.

Could I also say o seu irmão instead of o irmão dela? What’s the difference?

You could say o seu irmão, but it doesn’t automatically mean her brother:

o seu irmão can mean:
- your brother (talking politely to você / o senhor / a senhora), or
- his brother, or
- her brother, or
- their brother, depending on context.

Because seu/sua is ambiguous, Portuguese speakers often prefer:

o irmão dela = her brother
o irmão dele = his brother

These forms are very clear and natural in both spoken and written European Portuguese.

Why is the subject pronoun ele missing before quer? Shouldn’t it be Ele o irmão dela quer… or Ele quer…?

The subject here is o irmão dela, so we do not need ele:

O irmão dela quer trabalhar… = Her brother wants to work…

You generally do not repeat the subject with a pronoun right after a noun subject in Portuguese:

✅ O irmão dela quer trabalhar.
❌ O irmão dela ele quer trabalhar. (wrong / redundant)

You could start a sentence with the pronoun instead of the noun:

Ele quer trabalhar em tecnologia médica. (He wants to work in medical technology.)

But then the context must already make it clear that ele refers to o irmão dela. In the original sentence, the noun phrase o irmão dela is doing the subject’s job.

How does quer trabalhar work grammatically? Why no extra word like “to” or “a”?

Portuguese doesn’t use a separate word like to before infinitive verbs.
The pattern is simply:

quer (3rd person singular of querer) + infinitive

So:

quer trabalhar = wants to work
quer estudar = wants to study
quer viajar = wants to travel

No preposition is needed here. It’s just [querer] + [infinitive].

Why is it trabalhar em tecnologia médica and not trabalhar na tecnologia médica or with another preposition?

With areas of work or study, trabalhar em + área is very common:

trabalhar em tecnologia médica = to work in medical technology
trabalhar em educação = to work in education
trabalhar em finanças = to work in finance

You could also say:

trabalhar na área de tecnologia médica = to work in the field of medical technology

But trabalhar em tecnologia médica is shorter and perfectly natural.

Na tecnologia médica (with the article) would usually sound like a very specific sector or context already known in the conversation; for a general field, em tecnologia médica is more usual.

Why is it tecnologia médica and not tecnologia médico?

In Portuguese, adjectives agree in gender and number with the noun they describe:

a tecnologia (feminine singular)
médica is the feminine singular form of the adjective médico/médica.

So:

tecnologia médica = medical technology
- feminine noun (tecnologia)
- feminine adjective (médica)

If you said tecnologia médico, the adjective would be in the masculine form and would be grammatically wrong.

What is the difference between mas and mais? They look similar.

They are completely different words:

mas (with s) = but
- …, mas gosta muito de ciência pura. = …, but he really likes pure science.
mais (with is) = more
- Ele quer ganhar mais dinheiro. = He wants to earn more money.

They also sound different in European Portuguese:

mas – short, like “muss”
mais – sounds like “mice” (English word)

Why do we say gosta de ciência pura and not just gosta ciência pura?

In Portuguese, the verb gostar almost always needs the preposition de before its object:

gostar de
- noun:
- gosta de música = (he/she) likes music
- gosta de ciência pura = likes pure science
gostar de
- infinitive verb:
- gosta de ler = likes to read
- gosta de trabalhar = likes to work

Saying gostar directly followed by a noun (gosta ciência) is incorrect.
So de is required: gosta de ciência pura.

Could we say gosta muito da ciência pura instead of gosta muito de ciência pura? Is there a difference?

Both are possible, but there is a nuance:

gosta muito de ciência pura
- More general: he likes pure science as a field, in general.
gosta muito da ciência pura
- Often sounds a bit more specific or definite, as if:
  - referring to some particular pure science previously mentioned, or
  - treating pure science as a more clearly defined concept.

In everyday speech, when talking about liking a subject in general, people very often omit the article:

gosta de música, de futebol, de matemática, de ciência pura.

So the original gosta muito de ciência pura is the most neutral and natural way to say this.

Why is muito placed after gosta in gosta muito de? Can I say muito gosta de?

The normal position for muito (meaning very much / a lot) is after the verb:

gosta muito de ciência pura = he likes pure science very much / a lot.

Putting muito before the verb (muito gosta de…) is unusual in normal speech and would sound poetic or very emphatic, not natural in everyday Portuguese.

So you should stick to:

gosta muito de…
quer muito trabalhar em…
estuda muito

Why is it ciência pura and not pura ciência? Can adjectives go before the noun?

The regular position for adjectives in Portuguese is after the noun:

ciência pura = pure science
tecnologia médica = medical technology
livro interessante = interesting book

Adjectives can sometimes appear before the noun, but that is less common and often carries a stylistic or meaning change (more emotional, poetic, or idiomatic):

pura ciência could be understood in some contexts, but here it would sound strange or at least marked in European Portuguese.

In neutral, descriptive sentences like this one, you almost always put the adjective after the noun: ciência pura.

How is this sentence pronounced in European Portuguese?

Approximate pronunciation with English-like hints (not exact, but close):

O irmão dela → oo ir-MÃU DEH-lɐ
- irmão: final ão is a nasal sound, roughly like “aun” in French “faune”, not like English “ow”.
quer trabalhar → kehr trɐ-bɐ-LYAR
- quer: like “kehr” (short e).
- trabalhar: stress on the last syllable: tra-ba-lhar (the lh is like the lli in “million”, but shorter).
em tecnologia médica → ẽyng tek-nu-lu-ZEE-ɐ MÉ-dik-ɐ
- em: nasal, like “e” with your nose engaged; often sounds close to “ẽyng”.
- médica: stress on mé, final -ca is like “kɐ”.
mas gosta muito → muss GÓSH-tɐ MOYN-tu
- gosta: gósh-tɐ, with a soft “sh”-like s.
- muito: in European Portuguese, often close to “moy-tu”.
de ciência pura → dɨ see-ÉN-syɐ POO-rɐ
- de: often a very reduced sound, almost like a quick “dɨ”.
- ciência: stress on én: ci-ÉN-cia.
- pura: POO-rɐ.

Spoken naturally, it all flows together quite quickly:

O irmão dela quer trabalhar em tecnologia médica, mas gosta muito de ciência pura.