一天比一天好: better and better, day by day
【四29】固定格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
一 + measure word + 比 + 一 + measure word + adjective says that a quality keeps growing step by step: each day, each time, each year goes beyond the one before. 天气一天比一天暖和: the weather gets warmer day by day; 他的汉语一年比一年好: his Chinese improves year after year. The same measure word is repeated on both sides of 比: 天, 周, 年, 次, 个, 双…
Each item is compared with the one just before it, as if you were counting (day one, day two, day three…). That is what sets it apart from 越来越 ("more and more", a smooth trend with no counting) and from the plain comparison A 比 B + adjective. As in every 比 sentence, the adjective stands alone: no 很, no 更. With a verb, keep the degree complement: 一次比一次跑得远.
With 个 or 双 the sentence compares people or things lined up one after another: 这些孩子一个比一个高 (each taller than the one before). Neutral register, very common in speech and writing. Use it for a change that keeps going (better, colder, dearer), not for a fixed state.
Examples
天气一天比一天暖和了。
Tiānqì yì tiān bǐ yì tiān nuǎnhuo le.The weather is getting warmer day by day.
他的汉语一年比一年好。
Tā de Hànyǔ yì nián bǐ yì nián hǎo.His Chinese gets better year after year.
你看，这几个孩子一个比一个高。
Nǐ kàn, zhè jǐ ge háizi yí ge bǐ yí ge gāo.Look at these kids: each one is taller than the last.
他的演出一次比一次精彩。
Tā de yǎnchū yí cì bǐ yí cì jīngcǎi.Each of his shows is better than the last.
这个星期，我睡得一天比一天晚。
Zhège xīngqī, wǒ shuì de yì tiān bǐ yì tiān wǎn.This week I go to bed a little later every day.
这些球鞋一双比一双好看，我不知道买哪双。
Zhèxiē qiúxié yì shuāng bǐ yì shuāng hǎokàn, wǒ bù zhīdào mǎi nǎ shuāng.Each pair of sneakers is nicer than the one before; I don't know which to buy.
Common mistakes
✗ 天气一天比一天很暖和 → ✓ 天气一天比一天暖和 (like any 比 sentence, this pattern does not take 很).
✗ 一天比一年好 → ✓ 一天比一天好 (the same measure word on both sides of 比).
✗ 一天比一天越来越冷 → ✓ 一天比一天冷 / 越来越冷 (both already mean "more and more": keep only one).