A vocabulary list is a useful, honest tool, and it is also a flattened one. It gives you a word and a translation, one line each, as if every word in Korean behaved the way most English words do: fixed, neutral, usable in any sentence you drop it into. Four features of Korean make that flattening cost more than it does in most languages, and it is worth knowing about them before they surprise you mid-conversation.
1. Two number systems, and native speakers never confuse them
Korean keeps a native counting system alongside one borrowed from Chinese, and which one you reach for depends on what you are counting. Age, hours on a clock, and objects you count one by one generally take the native numbers. Minutes, dates, money, phone numbers and measurements generally take the Sino-Korean set. A learner who has only memorised "the word for three" has memorised half a fact. The other half — which "three" to say, and when — is not something a translation column can carry, and it takes genuine exposure to sentences, not just words, before it stops requiring conscious thought.
2. A word does not come with a politeness level attached
Korean marks the relationship between speakers directly in the language itself, through verb endings and, for a smaller set of words, through entirely different vocabulary for the same idea depending on who you are addressing and how you stand relative to them. The dictionary form a word list gives you is a citation form, not something you would necessarily say out loud to your professor, your younger cousin, or a stranger on a train. This is not a footnote to Korean grammar; it is close to the centre of it. A vocabulary list can teach you the word for "eat". It cannot teach you which of several verbs meaning "eat" belongs in front of your grandmother, because that is a fact about the conversation, not about the word.
3. Chinese-origin syllables are leverage, once you notice them
A large share of Korean vocabulary, especially the more formal, technical and abstract end of it, is built from syllables that originally came from Chinese, the way a large share of formal English vocabulary is built from Latin and Greek roots. The syllable 학 (hak), meaning study or learning, turns up in 학교 (hakgyo, school), 학생 (haksaeng, student) and 대학 (daehak, university). Once you can spot a handful of these recurring syllables, unfamiliar words stop being opaque strings and start being combinations you can half-guess, in exactly the way an English speaker can guess at "biology" from "bio" and "logy" without ever having seen the word before. This is one of the few places where Korean vocabulary gets easier as it gets more advanced, rather than harder.
4. The word on the page is not quite the word you will hear
Hangul spells each syllable block fairly consistently, but Korean applies a set of sound-change rules across the boundary between blocks, so the spoken word can diverge from what the spelling suggests. The old kingdom name 신라 is spelled with an "n" and an "l" side by side, and is pronounced "Silla", because that combination of sounds shifts in speech. 좋아요 is spelled with an "h", and many speakers pronounce it closer to "jo-a-yo" than "joh-a-yo". None of this is irregular or rare; it is a small set of rules that apply constantly, and it is the reason a learner who has only ever read Korean can struggle to follow the same words spoken at normal speed.
None of the four things above is something a widget can decide for you — they depend on the sentence and the room you are standing in. What Watch Your Language can do is keep the words themselves in front of you, in Hangul with a romanisation alongside, a few at a time on your iPhone Lock Screen and Apple Watch face, so the vocabulary is not the part still slowing you down when you meet these rules for real.
What this means for how you study
None of these four things argue against building vocabulary early and often; they argue against expecting a word list, on its own, to teach you how to use what is on it. Learn the words. Then learn them again inside sentences, spoken aloud, at normal speed, to whoever will have the conversation with you — that is where the number system, the politeness level and the pronunciation actually live.
