Several parts combine into one character.

下 is 上 flipped: a long horizontal baseline with a shorter mark below. Together, 上 and 下 form one of the oldest matched pairs in the script — twin indicative characters that use spatial position to encode position itself. Identical across 繁體 / 新字体 / 简体.

Mandarin: xià, falling 4th tone. 下 covers physical "down", verbal "go down / get off", temporal "next / later", and ranking "inferior": 下面 (xiàmiàn, below), 下车 (xià chē, to get off a vehicle), 下班 (xià bān, to leave work), 下次 (xià cì, next time), 下午 (xiàwǔ, afternoon — "after noon"), 下星期 (xià xīngqī, next week), 一下 (yīxià, "for a moment / once" — softening particle: 看一下 = "take a quick look"). The "next" sense (下次, 下星期) is grammatically essential and deserves separate memorization from the spatial sense.

Japanese splits the on-reading dramatically by context, more than almost any other character. カ (ka) appears in compounds like 地下 (chika, underground), 部下 (buka, subordinate), 下車 (gesha — wait, this is げ!), 以下 (ika, below/under). ゲ (ge) takes over in 下手 (heta, unskillful — "lower hand", and irregular reading), 下車 (gesha, getting off), 下品 (gehin, vulgar), 下宿 (geshuku, lodging). The split is partly historic and partly meaning-based — there is no clean rule, you must memorize compound by compound.

Kun-readings: した (shita, below — spatial), しも (shimo, lower part — abstract/social), もと (moto, foundation/under — old usage), さ.げる (sa.geru, to lower), く.だる (ku.daru, to descend), お.りる (o.riru, to get off / go down). At least six distinct verbs orbit this single character.

Memory aid: a baseline with a tick below — the inverse of 上.