"Our friend rides in a car."
Translation:우리의 친구가 자동차를 탑니다.
My wife is Korean and uses this form all the time. I've been forcing myself to learn the conjugated forms because I'll need to know them eventually (e.g. to talk to my in-laws), and I'm not sure if the -다 forms are actually grammatical or just common (i.e. whether they're slang). I'll most likely use them more as I get more confident in the language.
My boyfriend is also Korean and that's where I get my info as well! I do think it's grammatical. For some reason I think it might actually be somewhat academic although I don't remember where I got that. NOTE: I just googled it and this article is saying there's a declarative ending ~다 and it says it's a book style (academic I think) which would seem in line with what my boyfriend I believe has mentioned before. That just seems odd since I would think it would be 100% the same as the infinitive. I'll talk to my boyfriend and my Korean teacher and come back with more information later!
I think you guys are referring to the plain form. It is used in newspapers, books, etc. The speech level you use basically depends on the age of the listener. But In a news article you dont know the age of the person who will be reading it, so they use this plain form. It also can be used in speech. I think this video explains it better https://youtu.be/6JAoev3gH0E
I always get sentences wrong because I apparently either use 가/이 or 는/은 incorrectly. From what I have learned and looked up so far, though, the use of these partices highly depends on the context and what one wants to express. Should I report it if I am marked wrong because of these particles or is there really only the one answer to most of the sentences on here?
Particles can change the meaning of a whole sentence. I would avoid reporting sentences if you get them wrong because of particles because the particle really may be necessary for the meaning. Even if it's not necessary (and you could drop it in everyday speech), being exposed to the particles over and over should help you come to understand when to use each particle for those times when you do want or need to know the particles.