You should only use the indefinite article when you're singling someone out of a pool of professionals in a given area, usually with the purpose of making a personal characterization about this individual:
- Ela é uma engenheira militar de sucesso. She's a successful military engineer.
- Ela é uma má professora. She's a bad teacher.
- Ele é um trabalhador competente. He's a competent worker.
- Ele é um político sem escrúpulos. He's an unscrupulous politician.
Very interesting. An observation one could make here is the adjective present in all your examples qualifying the noun of profession. I'm curious why militar didn't work on its own and would have thought "Ela é uma engenheira militar" highlighted the fact that she is not just an engineer but one that works in the military, even though "Ela é engenheira militar" works just fine. Is this one of those differences between EP and BP? Thanks!
Sure, I translated the sentence"Ela é engenheira militar." To the literal " She is military engineer" I was going to put "She is A military engineer" but in fear of getting it wrong I left the "a" out and I got it wrong. I think if it wanted the article "a" it should have been left in "uma" inthe original sentence
Well, the shortest answer is no - you're required to use the article in English at all times, but we don't use the article in Portuguese when you're introducing your profession; basically that means that the proper equivalent of "Ela é engenheira" is "She is an engineer" and will always be so - that's just how each language works.
P.S. You can compare this situation with the examples I gave above where um/uma are actually employed with professions in Portuguese.
Good luck with your studies :)