This is a really great question though and something I really struggle with, so I'm going to explain it further. Sean is correct. I think of it like this: Adjectives work the opposite way in Korean. To be brief, in Korean when you look up descriptive words (adjectives) in the dictionary (so in their standard form) they are always verbs. In Korean, the "to be" is always attached. However in English, these descriptive words are just adjectives already. If you want an adjective to be a verb, you need to add "to be". SO in Korean the default is "to be cute" and in English the default is "cute".
With this in mind, it makes sense that if going from adjective -> verb form requires adding something in English, the reverse is true in Korean. To take a descriptive verb (to be cute) into an adjective (cute) we must add something. In Korean it is called a modifier. The Korean adjective modifier is ~(으)ㄴ. So 귀엽다 (to be cute) becomes 귀여운 (cute).
In your sentence "The boy reading a book is cute" You have used cute in its "to be" form which would be 책을 읽는 귀여운 남자아이 귀여워요. Do you see the difference? :)
The last thing I will add is you can use this adjective modifier with action verbs as well, which is what is used in the first half of the sentence "책을 읽는" this is literally making "book reading" an adjective. You could think of this sentence "책을 읽는 귀여운 남자아이" as literally "The cute, book-reading boy", as mentioned above in an earlier comment. But if this last paragraph confuses you, ignore it. Hope this was helpful.
Talk to me in Korean Level 3 lessons 13 and 14 cover adjectives in Korean for both descriptive verbs and action verbs.
it actually sounds more like ingnun because of the ㄱ + ㄴ assimilation. See the notes on the first lesson. Many consonants change their pronunciations when a consonant at the end of one syllable influences or is influenced by the consonant at the start of the next.
(1) The given example is a simple phrase with no verb.
책을 읽는 (adj) || 귀여운 (adj)|| 남자 아이 (noun)
= (who is) reading book || cute || boy
= a cute boy reading a book.
(2) Your suggestion: "A cute (adj) || boy (noun) || is reading (verb) a book" is a full sentence with Adj + Subject + Verb.
= 귀여운 (adj.)|| 남자아이가 (subject noun) || 책을 읽는다 (verb)
-> different grammar structure to the given example. Reason you were marked wrong.
There is no verb in the example. It’s not a complete sentence. See comment above: https://forum.duolingo.com/comment/24402887?comment_id=35844622
why is it 읽는 and not 읽은?
- 읽은 is the past determiner
- 읽는 is the present determiner
- 읽을 is the future determiner
It ends with a consonant so it should be 은 no?
Compare with 덮다 which also ends in a consonant:
- 덮은 is the past determiner
- 덮는 is the present determiner
- 덮을 is the future determiner
Compare with 느끼다 which does not end in a consonant:
- 느낀 is the past determiner
- 느끼는 is the present determiner
- 느낄 is the future determiner
Compare with 날다 which technically ends in a consonant but has special rules for -ㄹ (deletion):
- 난 is the past determiner
- 나는 is the present determiner
- 날 is the future determiner
Compare with 잇다 which technically ends in a consonant but also has special rules for -ㅅ (silence):
- 이은 is the past determiner
- 잇는 is the present determiner
- 이을 is the future determiner
The 는 part doesn’t change for the present determiner. But the past and future determiners change form depending on the root.