"The bread is small."
Yes, so there are markers in Korean. There's lots of em. Two of them are 이/가 and 은/는. While both of these work to specify the subject of the sentence, 은/는 is general and 이/가 is particular. And apart from that, 이/가 are both the same. The only difference is that 이 is used with words ending in consonants and 가is used with words ending in vowels. And same for 은/는, 은 is for words ending in consonants and 는 foe vowels. As an example- 여자가 Would mean 'the woman' and 여자는 would mean 'a woman' :)
Here's some more info on those markers
Think of 가 and 이 as being used to bring in new information, and 은/는 being used to connect what's already known to the new information.
In this sentence: 남자는 메시지가 있습니다 (The man has a message), you probably would've already known about the man with past context.
Maybe you're sitting in your office and your secretary comes in. "There's a man outside wanting to see you." They say. There would've likely been a 가 attached to the man, since it's new information that he exists.
"What does he want?" You reply.
"He (the man) has a message./남자는 메시지가 있습니다." The secretary replies. You already know about the man. He's not new information. What's new information is the message. So, 'message' is the item that will have 가 attached to it, putting more emphasis on it than the man. The man will just have 는 attached to it to attach it to the next word.
I learned this concept from Japanese, and from what I've seen so far, it seems to be the same in Korean. I never understood it when people just said "as for (item), etc. etc." when explaining は, which in Korean is 은/는, and it seems that sentence is popping up here too. I didn't even know what that meant! As for the man? How does "as for..." tell me when to use は or が (은/는 or 가/이)?
I feel like a better way to explain it would be "as for (object), which you already know about +은/는, this is what's new that exists +가/이."
So, to connect that back to the original sentence 남자는 메시지가 있습니다, it'd be like this:
"As for the man (which you already knew about, so you'd use 는 with him), he has a message (using 가 since this is new information, so it gets more emphasis)."
Of course, if you're introducing the man and the message in the same sentence, you'll just put 가/이 on whatever needs more emphasis or could be considered more important.
남자는 메시지가 있습니다. There is a man and he has a message, but the part with more attention/emphasis is the message he has.
남자가 메시지는 있습니다. (I switched 가 and 는). There is a man and he has a message, but what's getting more attention is that there's a man that has the message. 》Maybe the secretary from before came in and said "There's a message for you," making the message already known. "Who has it?" You ask. "A man has the message," replied the secretary, placing the 가 on the man because he's the new information.
Hopefully this makes more sense to anyone reading it! I know I could've used a better explanation when I first learned how this stuff worked, heheh.