A cientista trabalha no laboratório.

A is the feminine singular definite article, meaning “the”.

So:

• a cientista = the (female) scientist
• It tells you the noun is feminine and singular, and that we are talking about a specific scientist, not just any scientist.

Yes.

• o cientista = the (male) scientist
• a cientista = the (female) scientist

The noun cientista itself doesn’t change; the article (o / a) marks the gender.

In Portuguese:

• cientista is a noun = scientist (a person)
• científico / científica is an adjective = scientific (describing something)

So:

• a cientista = the scientist
• artigo científico = scientific article / paper

You need the noun for a profession, so cientista is correct.

In Portuguese, a singular countable noun (like cientista) almost always needs some kind of determiner (article, demonstrative, etc.):

• A cientista trabalha… – natural
• Cientista trabalha… – sounds wrong in standard Portuguese

You can drop the article with professions in specific patterns like:

• Ela é cientista. – She is a scientist.

But when the profession is a clear subject of a full sentence with more information, you normally keep the article:

• A cientista trabalha no laboratório.

• trabalhar is the infinitive = to work

• trabalha is the present tense, 3rd person singular form of trabalhar:

  • (ela) trabalha = she works / is working
  • (ele) trabalha = he works / is working

In A cientista trabalha…, a cientista is the subject, so the verb must be conjugated as trabalha.

Portuguese is a “null-subject” (or “pro-drop”) language: you can usually omit subject pronouns because the verb ending and the context show who is doing the action.

• Ela trabalha no laboratório. – She works in the lab.
• Trabalha no laboratório. – still She works in the lab (if context is clear)
• A cientista trabalha no laboratório. – subject is clearly a cientista, so no pronoun is needed.

The pronoun ela would sound redundant here.

In Portuguese, the simple present often covers both:

• A cientista trabalha no laboratório.
  • can mean The scientist works in the lab (in general, habitually)
  • and in some contexts also The scientist is working in the lab.

If you want to emphasize right now in European Portuguese, you typically use:

• A cientista está a trabalhar no laboratório. – The scientist is working in the lab (right now).

(Compare with Brazilian Portuguese: está trabalhando.)

no is a contraction:

• em (in / at) + o (the, masculine singular) → no

So:

• no laboratório = em + o laboratório = in the / at the laboratory

Because laboratório is masculine:

• masculine singular: o laboratório → no laboratório (in the laboratory)
• feminine singular: a sala → na sala (in the room)

So:

• em + o → no (for masculine nouns)
• em + a → na (for feminine nouns)

Use the indefinite article:

• um laboratório = a laboratory
• em + um laboratório → num laboratório

So:

• A cientista trabalha num laboratório.
  = The scientist works in a laboratory (in some lab, not a specific one already known).

Approximate IPA and a rough English-based guide (for European Portuguese):

• trabalha → /trɐ.ˈba.ʎɐ/

  • tra-: like “truh”
  • -ba-: like “bah”
  • -lha: the lh is like the ll in Spanish llama or similar to the lli in English million; ends with a schwa-like sound.

• laboratório → /lɐ.bu.ɾɐ.ˈtɔ.ɾju/

  • la-: “luh”
  • -bo-: “boo”
  • -ra-: “ruh”
  • -TÓ-: stressed, like “taw”
  • -rio: roughly “ryu”, with a quick r and a light final vowel.

(The r is a single tap [ɾ] in the middle of the word in standard European Portuguese.)

In Portuguese, written accents mainly show:

1. Where the stress falls (which syllable is stressed).
2. Sometimes, which vowel quality to use (open vs closed).

Without knowing the rules, a word ending in -io might be stressed on the penultimate syllable. The accent on ó in laboratório tells you:

• The stress is on tó: la-bo-ra-TÓ-rio
• And that this vowel is pronounced as an open “ó” sound (/ɔ/).

So the accent guides both stress and sound.

Pluralize both the noun, the article, the verb, and (optionally) the location:

• As cientistas trabalham nos laboratórios.

Breakdown:

• a cientista → as cientistas (the scientists, feminine or mixed if context says so)
• trabalha → trabalham (they work)
• no laboratório → nos laboratórios
  • no = em + o → nos = em + os
  • laboratório → laboratórios

Yes, that is correct and natural.

• A cientista trabalha no laboratório. – neutral, subject-first.
• No laboratório, a cientista trabalha. – also correct; puts a bit more focus on the location (“In the lab, the scientist works”).

Portuguese word order is fairly flexible, especially when you move prepositional phrases like no laboratório.

No.

In Portuguese:

• Professions and roles are normally not capitalized, unless they start a sentence.

So you write:

• a cientista, o médico, a professora, o engenheiro
  not A Cientista, O Médico, etc.