A word referring to an adult male
When to use
Adult male reference, Boyfriend expression, Men's restroom sign
The nuance
Used to refer to adult males; not used for young boys.
남자 (nam-ja, namja) · 여자 (yeo-ja, yeoja) · 아이 (a-i, ai)
남자 (man), 여자 (woman), and 아이 (child) are basic nouns distinguishing people by gender and age, requiring precise usage depending on context.

One lane per word — no nested boxes, just the gist.
A word referring to an adult male
When to use
Adult male reference, Boyfriend expression, Men's restroom sign
The nuance
Used to refer to adult males; not used for young boys.
A word referring to an adult female
When to use
Adult female reference, Girlfriend expression, Women's restroom sign
The nuance
Used to refer to adult females; not used for young girls.
A word referring to a young child, used regardless of gender
When to use
Referring to young age, Daycare or kindergarten context, Distinguishing from adults
The nuance
Used to refer to young children regardless of gender. Not used for 성인 (adults).
그는 훌륭한 남자다.
geu-neun hul-ryung-han nam-ja-da., geu neun hul ryung han nam ja da.
Correct use referring to an adult male.
그 남자는 5살이다.
geu nam-ja-neun 5sal-i-da., geu nam ja neun 5sal i da.
At 5 years old, he is a boy (아이), not an adult man (남자).
여자는 학교에 간다.
yeo-ja-neun hak-gyo-e gan-da., yeo ja neun hak gyo e gan da.
Natural sentence referring to an adult female.
그 여자아이는 예쁘다.
geu yeo-ja-a-i-neun ye-ppeu-da., geu yeo ja a i neun ye ppeu da.
Correct use combining '아이' and '여자' for a young girl.
아이 남자가 축구를 한다.
a-i nam-ja-ga chuk-gu-reul han-da., a i nam ja ga chuk gu reul han da.
'아이 남자' is unnatural; correct is '남자아이'.
아이들이 놀고 있다.
a-i-deul-i nol-go iss-da., a i deul i nol go iss da.
Natural expression referring to multiple children.
여자아이는 학교에 간다.
yeo-ja-a-i-neun hak-gyo-e gan-da., yeo ja a i neun hak gyo e gan da.
Correct expression referring to a young girl.
남자아이가 공을 던진다.
nam-ja-a-i-ga gong-eul deon-jin-da., nam ja a i ga gong eul deon jin da.
Correct expression referring to a young boy.
Which word correctly refers to a 5-year-old boy?
Which word is appropriate for an adult female?
What is the correct term for a young boy?
Which word indicates an adult male?