Korean has different types of markers:
•case markers s.a. 이/가 (subject); 의 (of); 에 (at, in); 을/를 (object); 에게 (indirect object)...
•auxiliary markers s.a. 은/는 (topic); 도 (also/even); 만 (only); (이)나 (or) ...
Case markers act as role indicators of the attached words. In many cases, these roles are clear from the context of the sentence and these markers can be dropped, especially in daily speech (note: English does not need subject/object markers). Honorific markers (께서/께) cannot be omitted as they also mark respect.
Aux. markers on the other hand add meanings to the sentence (These are mostly translated as adverbs or adverbials in Eng.: as for; only; even; too...) and should not be omitted.
When used with weak case markers like 이/가, 을/를, aux. markers can override the latter (in the exception of honorifics).
Do not need subject/topic/object markers in this Korean sentence.
+만 = "only": serves here as a suffix (English language term), a particle added to end of noun.
In this sentence: No need for other particles (suffices), or articles before word nor prefixes (English term for things attached to front of word.
▪I only eat these side dishes/I eat these side dishes only = 이 밑반찬(을)만 먹어요 => -만 as sentential adverb modifies the whole sentence 이 밑반찬 먹어요 eg.
이 반찬(을)만 먹지만 만들지는 않아요 I only eat these side dishes, but I don't make them.
▪I eat only these side dishes = 이 반찬만을 먹어요 => -만 as componential adverb modifies the noun 반찬 only. eg
이 반찬만은 먹고 매운 것은 안 먹어요. I eat only these side dishes and not the spicy ones. => 은 replaced 을 in this case for contrasting purpose and to put emphasis on 반찬.
I only eat the spicy side dishes = 매운 밑반찬(을)만 먹어요
I eat only the spicy side dishes = 매운 밑반찬만을 먹어요
저만 이 반찬을 먹어요 = Only me, I eat these side dishes (ie I am the only one eating these side dishes).
Present continuous is actually correct when speaking of ,,today"
Check this link: https://www.thoughtco.com/present-continuous-tense-1211150
The present continuous tense expresses something that is happening at the moment of speaking. It is frequently used in conjunction with time expressions such as "right now" or "today" to indicate that an action is occurring at that moment.
However, present simple mostly indicates repeating acts and ,,Today I'm only eating side dishes" isn't a repeating act
Yeah, you wouldn't normally say "today I eat...", it should be "today I am eating..." in English. I don't know if the problem is Duolingo, a verb form that has to be translated without continuous implication, or if it's that 오늘 doesn't quite line up with the English-language concept "today".