一定
yīdìng · HSK 2
must
一 (one) + 定 (fixed) = 'one fixed thing' → definitely, must
定 means fixed/settled, so 一定 literally reads as 'one settled matter' — something locked in, certain. English gloss: definitely, surely, must. Common in promises like 我一定来 (I will definitely come).
一 — single horizontal stroke
一 is one of the simplest pictographs in Chinese, depicting a single horizontal line to represent the number one. It has retained essentially the same form since oracle bone script over 3,000 years ago. By extension it also means 'first,' 'whole,' or 'same.'
定 — roof 宀 + 正 (correct/upright, phonetic)
Originally written with 宀 (roof) over 正 (upright/correct), suggesting a household settled and orderly under its roof. Over time 正 simplified to 㐬-like form at the bottom, and the meaning shifted from 'settled home' to the broader senses of 'fix, stable, decide.'