亮 liàng

Say 漂亮 with a full fourth tone on 亮 and people will hear a foreigner reading a word list. Real speech flattens it: piào liang, yuè liang. The stress in those words sits on the first syllable, so 亮 gets crushed down to a quick, toneless liang. Now look at 天亮. Here 亮 is the whole point of the sentence, so it keeps its full fourth tone: tiān liàng. Same character, two jobs: load-bearing word versus tag-along ending.

English 'bright' does two jobs: a bright lamp, a bright kid. Chinese keeps them apart. 亮 covers light, glow, shine — lamps, screens, moonlight, the morning sky. Brains get 聪明 (cōngming). Tell a Chinese friend 他很亮 and you're saying he emits light. Expect a pause, then a laugh. Also notice 亮 leans on something else: 灯很亮, 天亮了, 月亮. 亮 by itself is a description, not a compliment.

Chinese sentences put the thing being talked about up front, then say what happened to it. 天 is the thing. 亮 is what it did. So you get 天 + 亮. Flip it to 亮天 and you've built a label, 'a bright sky', which nobody uses to announce dawn. Add 了 and you get the everyday version people actually shout in the morning: 天亮了. That 了 is the 'it just happened' marker — the sky wasn't bright, now it is.