"예, 빵은 음식입니다."
Translation:Yes, bread is food.
In the most basic terms, the topic (는/은) is the general thing you want the conversation to be about for the moment, the umbrella under which the sentence is presented. Meanwhile, the subject (가/이) is the thing performing the action of your sentence, in grammatical terms it is most commonly the agent of a clause.
For example: "Speaking of school, I'm going back." 'School' there might be the topic of the sentence, and 'I' would be the subject.
A topic is the part of the sentence that has the most importance to you. As if we said "As for bread, yes, it is food." It is being marked as the most important part of the sentence. The topic could be the subject or the object. You are right that in this case the topic that is marked does happen to be the subject. If the topic marker is used on the subject, that word won't have a subject marker.
A subject does a verb. A topic is something a sentence is about. It could be a subject, an object, or something else entirely. "I-topic coffee-am" is a grammatical sentence, but it obviously doesn't mean that you are coffee, just "She'll take tea. As far as I am concerned, I will take coffee."
(sorry I know this is a year old) but why does it seem interchangeable? In this one, bread is the topic and the sentence translates to "bread is food", but in the last one bread was the subject and that sentence ALSO translated to "bread is food" ("bread" being the subject and "is" - - presumably - - being the verb).
If they can both be used there, how are they different? At first I thought "topic" meant "object" but I realize now that this isn't true
Both subjects of sentences and objects can be topics. http://organickorean.com/advanced-topic-marker-%ec%9d%80%eb%8a%94-vs-subject-marker-%ec%9d%b4%ea%b0%80/
Also, a subject marker is not only used on the subject of a sentence. Where you have [something “is“ something], we say in English grammar that [subject “is” predicate nominative (which refers back to the subject)], but “is” acts like an equal sign at times and in some languages they say that there are two subjects in this same situation talking about the two nouns. There is only one item of food that we are talking about which happens to be bread. So, you may see either noun with a topic marker and the other with a subject marker. If the topic marker is on what in English we call the subject of the sentence. It is as though we said “As for bread, it is food.” If the topic marker is on what in English we call the predicate nominative (which is only after a form of the verb “to be”), then it is as if we said “As for food, it is bread.”
Also when you get a chance, study online where the tips and notes are available for Korean. Not all languages have those helps available from the app.
Otherwise, bookmark this for accessing all not just this one lesson. It gets updated now and then.
Yes, "food" is a mass quantity and not countable. We would have to say "a kind of food" as "bread" itself is treated as a mass quantity, but that would not be a translation of this sentence. We can say a loaf of bread or a bun or a slice of bread, etc., which again is not a part of this sentence. There are many kinds of bread, so it is really an entire category of food.