Pictograph: an arrowhead point or a narrow funnel — the shape of something sliding INTO a tight opening. Identical across 繁體 / 新字体 / 简体.

Critical confusion alert: 入 looks almost identical to 八 (eight). Both are two-stroke characters with two diagonal lines, but the geometry is reversed: — 入: two strokes meeting at the TOP point, opening downward. Direction: into a narrow opening. — 八: two strokes diverging from the TOP, opening downward and outward. Direction: splitting apart. In handwriting, beginners constantly confuse them. The mnemonic: 入 has the two strokes meeting AT the top (like an arrowhead pointing IN); 八 has them separating from the top (like legs walking apart).

Mandarin: rù, falling 4th tone. 进入 (jìnrù, to enter), 收入 (shōurù, income — "what comes in"), 输入 (shūrù, input / import), 加入 (jiārù, to join), 投入 (tóurù, to invest / throw in), 入口 (rùkǒu, entrance), 入门 (rùmén, beginner-level / "enter the gate"). The collocation 进入 (jìnrù) is the most common verb form because 入 alone feels formal in modern speech.

Japanese: on-reading ニュウ (nyū) for compounds — 入学 (nyūgaku, school enrollment), 入院 (nyūin, hospitalization), 輸入 (yunyū, import), 入場 (nyūjō, admission/entrance to a venue). Kun-readings: は.いる (hai.ru, to enter — intransitive), い.れる (i.reru, to put in — transitive). The pair 入る/入れる is one of the foundational transitive/intransitive verb pairs every Japanese learner masters.

入口 (iriguchi, entrance) — pure kun-reading compound — is signed at every Japanese building. Pairs with 出口 (deguchi, exit) from the previous entry.

Memory aid: an arrowhead point — converging into a target. Compare 入 (in) with 八 (eight, splitting apart) — and never confuse them again.