定
dìng · HSK 4
to fix; to set; to make definite; to subscribe to (a newspaper etc); to book (tickets etc); to order (goods etc)
宀 (roof) + 正 (correct) — settle things correctly under one roof
定 combines 宀 'roof/house' with a phonetic component derived from 正 'upright/correct'. The sense moves from 'settle a household' to 'fix, stabilise, decide'. Same root in 决定 juédìng 'decide' and 一定 yídìng 'definitely'.
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.'