"I write a message."
Translation:저는 메시지를 적습니다.
In English “lie” can mean “to be in a horizontal position” or ”to speak untruthfully”. Sometimes two words can turn out the same by chance and you have to tell from context which is meant. For Korean 쓰다 grammar sometimes also helps: 쓰다 “to write” is a (dynamic) verb, 쓰다 “bitter” is an adjective/stative verb, which means that their grammatical forms are sometimes different.
My take anyhow:
"I write a message." 저는 메시지를 적습니다. Here you're emphasizing that the message is written. It's just an object for the verb.
"I write a message." 저는 메시지가 적습니다. Here you're emphasizing that a message is what is getting written, as opposed to something else. It's not just a passive object, but directing the action to it, involved in what's happening to it.
저는 메시지가 적습미다 sounds flat out wrong to me. -이/-가 marks a subject of the verb, i.e. usually either the kne that does the action (for process verbs) or that is in the state in question (for state verbs/adjectives). So 메시지가 적습니다 would mean that the message is doing the writing, not that it is written.
Now sometimes you do fet subjects without -이/-가. Most importantly, whenever subject happens to also take either the topic marker -은/-는 or -도 "too, as well", then those particles will "eat" -이/-가 so you only get -은/-는/-도. For example to say "I also write a message" you say 저도 메시지를 적습니다. -도 replaced the -가 on 저. But if -이/-가 is present it always marks the subject, never an object.
Correct, the infinitive/dictionary form of both these words is 적다. However there are differences which ensure that there hardly ever is a chance to confuse them:
- 적다 “to write down, to jot down” is a transitive verb; it always needs an object (at least an implied one, even if it’s not overtly stated). 적다 “to be little/few” on the other hand is an adjective/intransitive stative verb, meaning it can’t take an object.
- All endings which treat adjectives/stative verbs differently from dynamic verbs will differentiate between these two. For example the conjunctional ending -은/는데 (somewhat like a weak “but” or “seeing as…”) is -는데 after dynamic verbs but -은데 after stative verbs/adjectives (-ㄴ데 if it ends in a vowel). So if you attach this ending to our two verbs, you get 적는데 “writes down, but …” and 적은데 “are few, but …”
- 적다 “to write down” obviously only makes sense if the subject is a human (or some other being capable of writing).
- 적다 “to be little/few” only makes sense if the subject is either a mass noun or a plural (although the plural could be implied of course since Korean plural markers are not obligatory). So if you’re sure your subject is singular, the predicate can’t be “to be little/few”.
Comparing is one common reason why you would want to use -은/는, that’s true, but it’s not the only one.
Basically -은/는 signals that this is the thing about which the rest of the sentence (and in fact possibly following ones as well) will be talking. The comparison thing is one extension of this, because for a comparison you will first give information about topic X and then (probably contrasting) information about topic Y.
-이/가 on the other hand simply marks the subject if it is not the topic (if the subject is also the topic, -은/는 will “eat” -이/가 because the two particles can’t be simultaneously applied).
So 저는 메세지를 적습니다 provides new information about me; 제가 메시지를 적ᄉᆖᆸ니다 provides information about some implied topic (for example a time frame such as “today”), probably one which carries over from earlier. Since we don’t have any context in Duolingo, both are perfectly fine translations of the English sentence here.