便于
biànyū · HSK 5
convenient for
便 = convenient + 于 = for/at; literally 'convenient for'
便 means convenient or easy, and 于 is a classical preposition meaning 'for' or 'at'. Together 便于 = 'convenient for [doing X]', used before verbs like 便于携带 (easy to carry).
便 — person 亻 + change 更
A semantic compound: 亻(person) beside 更 (to change/replace). The original sense was a person making things easier or more convenient by adjusting them. From 'convenient' it extended to 'then/thereupon' (biàn) and to bodily 'discharge' (as in 大便).
于 — two horizontal strokes 二 + vertical hook 亅
于 originated as a pictograph of breath or sound flowing outward, depicting exhalation through a passage. In oracle bone script it resembled a curved line showing motion or extension. Over time it became a grammatical preposition meaning 'at,' 'in,' or 'to,' losing its original concrete sense.