When ㅅ is a batchim (ends a syllable/jamo) it has a T sound. Same goes for ㄷ, ㅆ, ㅈ, ㅊ and ㅎ. ㅂ and ㅍ have a P sound (if the next jamo starts with a vowel, ㅂ keeps it B sound). ㅋ, ㄲ and ㄱ, K (ㄱ has a G sound when the next jamo starts with a vowel). ㅇ, ㅁ, ㄴ and ㄹ has normal sound as batchim.
I recommend search better about batchim, it's very important
The written coronal sounds will be most frustrating to a beginner when in the coda position of a syllable. These are: ㄴ, ㄷ, ㄸ, ㅌ, ㄹ, ㅅ, ㅆ, ㅈ, ㅉ, and ㅊ. The surface realization of these sounds can change greatly depending on context.
Examples: (using a 1-to-1 romanization scheme with hangeul pronunciation in brackets and the spelling simply bolded)
the field: 밭 (bat→bad) + 이 (i) → [바치] (bati→bachi)
input: 입 (ib) + 력 (lyeog) → [임녁] (iblyeog→imnyeog)
shilla (state): 신 (sin) + 라 (la) → [실라] (sinla→silla)
petal: 꽃 (kkoch→kkod) + 잎 (ip→ib) → [꼰닙] (kkochib→kkodnib→kkonnib)
first kiss: 첫 (cheos→cheod) + 키스 (kiseu) → [척키스] (cheoskiseu→cheodkiseu→cheogkiseu)
that is: 있 (iss→id) + 는 (neun) → [인는] (issneun→idneun→inneun).
similarly: 비슷 (biseus→biseud) + 하게 (hage) → [비스타게] (biseushage→biseudhage→biseutage)
Of course, this goes for other sounds too besides the ones I mentioned, but the other ones have fewer rules and fewer possibilities for sound changes. Words are morphemically spelled, so you are expected to internalize these contextual transformations.
The sounds don’t work like that in Korean. You have to forget that there are any [b] (ㅂ), [d] (ㄷ), [dʑ] (ㅈ), and [g] (ㄱ) sounds in Korean, because they are not the pronunciations of the base forms. Instead, the base forms are pronounced [p] (ㅂ), [t] (ㄷ), [tɕ] (ㅈ), and [k] (ㄱ). Romanizations use b, d, j, and g out of convenience since these letters would otherwise go unused.
To truly understand the difference between pairs like ㄷ and ㅌ, you must be aware that the Korean mind distinguishes only the presence of aspiration. ㄷ is not aspirated; there is no puff of air accompanying the pronunciation of this consonant. ㅌ is aspirated; there is a palpable puff of air accompanying the pronunciation of this consonant. But both ㄷ or ㅌ can be voiced (ㄷ as [d] and ㅌ as [dʱ]) between voiced sounds (those sounds when your vocal cords are supposed to vibrate).
ㅌ is an unreasonable choice to use as a final consonant, because there is no aspiration at the end of syllables, and no aspiration when linked to a following null-onset (without initial consonant) syllable. Take the English sentence A “cat is out” for example and their resulting pronunciation in speech:
- a-ca-di-zout (yes)
- a-ca-ti-sout (no)
The t in cat is not aspirated as you can see, but voiced. This is the same in Korean. The biggest mistake is assuming that there is somehow a 1-to-1 equivalence between Korean phonemes and English phonemes, because as you can see in English, even t (written) is not always t (pronounced)!
As for the use of ㅅ rather than ㄷ (which I also find more logical), it’s simple: Koreans sometimes perceive borrowed words’ [t] coda as having an [s]-like quality before null-onset consonants.
If it helps someone:
The transliteration (translating the sound, not the meaning) of "도넛" as "dunut" and "donut" are both accepted in the exercise.
Is "넛" a "nut" in Korean? And 도 (do) meaning "too"? So is "doughnut" can be heard in Korean also in "nut too"? (Just too understand the logics of the language)
Just concentrate on the Korean sounds, not on English spelling. Learn the Hangul alphabet using one of many excellent youtube courses. Adapt your keyboard to type in hangul, and never use romanization again after you have learned the basic Korean sounds. The romanization was just just to help you get an idea of what the Korean letters sound like. The g and the h have no sound even in English, so you certainly won't find them in Korean.