Could you be more specific? English is my second language (my mother tongue is Polish and we don't have articels in Polish at all) and up till now I thought that infinitive articles 'a' and 'an' are translated into Spanish as 'un/una'. Is there any rule that explain why in this particular translation one should add the article to the noun? And would this be a mistake if I translate this sentence vice versa using Spanish indefinite article 'un': "My boyfriend is a journalist" -> Mi novio es un periodista"?
This may be complicated for you! ;) Articles do not translate consistently between English and Spanish, and native English speakers (like me) have a difficult time with them. The rule in Spanish is that professions (journalist, doctor,professor, and the like) do not have an article unless they are modified by an adjective. So it is "Mi novio es periodista" but it would be "Mi novio es un buen doctor."
In English, we always use an article before a profession. He is a doctor, he is a good doctor, she is the best doctor.
The general rules for when to use un/una and los/las are difficult for many of us. You can Google the topic and find explanations, some are better than others, so look at a few and find one that explains things in a way you can understand.
Native English speakers that do not have a good grasp of English grammar can have a hard time learning Spanish -- so you may be learning Spanish and improving your English at the same time.
Generally, a word that ends in -ista can be feminine or masculine.
Painters paint paintings, cyclists cycle on [bi]cycles, bakers bake baked goods in bakeries, farmers farm on farms, programmers create programs by programming... I'm actually a little annoyed with Spanish right now. I got 'la televisión' wrong tonight, because I put the accent in the wrong place, and used 'el', which seemed the obvious choice. Two comments up I learned that professions ending in -ista can be feminine or masculine (which explains why that wasn't making sense). I also just found out you don't need an article for professions. I was promised Spanish really stuck to its rules, but since I started here on Duolingo a month ago, I've encountered anything but. It seems almost as bad as English to me, especially when you mix in conjugation and singular/plural changes, which greatly amplifies how much one needs to remember and pay attention to.
I agree, but every other lesson so far has required a word by word translation. So why is this suddenly adding in words that aren't there. Plenty of times el or la were included that didn't need to be there but if you forgot it, well then it's wrong. I'm not actually asking you for an explanation, the app just doesn't seem to know what it wants. Still good though.
There exists many exceptions in Spanish which reside outside the rules. And, yes, each item needs to memorized, individually. However, consider this. Over all, Spanish makes English look like a trash bin of wreckage in comparison. That is, as fsr as having and following consistent tight rules is concerned, anyway.
Usually, -ista professions are non-gender specific. You never have a 'dentisto', 'artisto', 'periodisto' etc. Yes, you memorize the rules individually, but for this rule, you would learn the exceptions to '-ista is not gender specific.' Frankly, I can't think of an exception, but sometimes my memory fails.