"The singer is thin."
Short answer: Some descriptive verbs have citation forms that end in 하다.
You are observing how Korean conjugates its verbs, specifically the conjugation for the formal speech level.
Action verbs and descriptive verbs have a citation form which is then conjugated to the appropriate speech level when used. For the formal speech level, we take the verb stem and add -ㅂ니다/-습니다. Note that the additional letters we add depend on whether the verb stem ends in a vowel or consonant.
In this sentence, the verb is 날씬하다 ("to be slender") and has the stem 날씬하. Therefore the formal form of this word is 날씬합니다.
Contrast this with the verb 작다 ("to be small"), which has the stem 작. Therefore the formal form of the verb is 작습니다.
It does roughly equate to the and you can use it in multiple instances. 그 Is used when the thing in question is far from the speaker but close to the listener. E.g 그 책은 재미있어요 The book is interesting/fun or 그것은 차가워요 The thing is cold.
There are 2 other variations
이 (The thing is closer to the reader than the listener)
저(The thing is neither close to the speaker or the listener like when pointing out directions)
Good observation. 하다 only carries the meaning of "to do" when used in verbs (DLG: active verbs). 하다 carries the meaning closer to "to be, to feel, to express" when used in adjectives (DLG: descriptive verbs). Consider these two examples:
- 피곤하다 = "to be tired" or "to express tiredness" rather than "to do fatigue (피곤)"
- 슬퍼하다 = "to be sad" or "to act as if (subject) is sad" rather than "to do sadness"
Yes. In Korean there are two parts of speech that serve as predicates to a complete sentence: 동사 (verbs) and 형용사 (adjectives). For example, 공부하다 ("to study") is a 동사, while 피곤하다 ("to be tired") is a 형용사.
As distinct parts-of-speech, these words require different grammatical structures in sentences and affect conjugations.
Some English-to-Korean textbooks use active verbs and descriptive verbs to discuss this distinction. But I find it more confusing since there are already Korean-grown alternatives available.