"저는 아이를 가르칩니다."
Translation:I teach a child.
This should help:
- 는 - Topic marker, used after a vowel e.g. 여자는 - A woman. Used when making a general statement e.g. An apple is a fruit.
- 은 - Topic marker, after a consonant e.g. 연필은 - A pencil.
- 가 - Subject marker, used after a vowel e.g. 학교가 - The school
- 이 - Subject marker, used after a consonant e.g. 연필이 - The pencil. Used when you're talking about something specific e.g. THE pencil is a thing.
Hope you find that useful.
You definitely could use 에게 to mark who you teach and use 를/을 to mark the subject that you teach them.
But you wouldn't do that if you wanted to simply say that you teach kids. For example, a regular elementary teacher teaches all subjects, so they might simply say that they teach kids.
It has nothing to do with whether or not it's a subject. The plural marker is just not obligatory. You can explicitly mark things as plural but more often than not you don't need to and plurality is simply guessed from the context. Also -들 tends to imply a not just plural but also definiteness: 아이들 would be a particular group of children that the listener already knows about, not just any random children.
If I’m not mistaken you can only use “to” if you also mention the subject you teach, right? “I teach English to foreign students” is fine, but just “I teach to foreign students” (without mentioning what you teach) is not. Without “to” both works (although if you mention both whom you teach and the subject of the class, the latter has to come after the former: “I teach (foreign students) (English)”. At least that’s how it is in Standard English as far as I know. But if you’re a native speaker of English and your dialect allows “I teach to a child” without mentioning what you teach, then feel free to report it.
In Korean if you have only one object (only the subject of the class or only the student(s)) then you use -을/-를. If you have both you use -에게 for the student(s).
- 저는 아이를 가르칩니다. “I teach a child/children.”
- 저는 영어를 가르칩니다. “I teach English.”
- 저는 아이에게 영어를 가르칩니다. “I teach English to the/a child/(the) children.”
That would indicate that Duolingo wasn’t able to identify which word you tried to write. Maybe there were too many typo’s in a single word, or maybe the mistake made the word too similar (or even identical) to a different word. For example typing 자 instead of 저 almost certainly would not be accepted because 자 is a different word, meaning “ruler (for measuring)”.