忽A忽B, 时A时B: now A, now B
【高64】【高66】固定短语 / phrases
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
忽A忽B and 时A时B describe a state that swings between two opposites: 忽高忽低 (now loud, now soft), 时好时坏 (now good, now bad). A and B are two bare one-syllable adjectives or verbs of opposite meaning, which gives the four-character rhythm that Chinese loves.
The nuance lies in the first syllable. 忽 stresses suddenness: 忽明忽暗 for a flickering light, 忽冷忽热 for weather or a body that jumps from one to the other. 时 stresses alternation over a period: 时快时慢, 时好时坏 for results that vary from month to month. Both are polished expressions, common in description and narration.
The group acts like an adjective or a stative verb: a predicate (里边的说话声忽高忽低), a complement after 得 (心跳得忽快忽慢) or a modifier with 的 (忽冷忽热的天气). It takes only two monosyllables; for longer pieces use 时而……，时而…… (level 6). Do not confuse it with 又……又……, which joins two qualities at the same time.
Examples
他的成绩时好时坏，老师有点担心。
Tā de chéngjì shí hǎo shí huài, lǎoshī yǒudiǎn dānxīn.His grades are now good, now bad, and his teacher is a little worried.
山里的天气忽冷忽热，出门最好多带一件衣服。
Shān li de tiānqì hū lěng hū rè, chūmén zuìhǎo duō dài yí jiàn yīfu.The weather in the mountains swings between cold and hot, so it is best to take an extra layer.
叔叔的书房门关着，里边的说话声忽高忽低。
Shūshu de shūfáng mén guān zhe, lǐbian de shuōhuà shēng hū gāo hū dī.The door of my uncle's study is shut, and the voices inside rise and fall.
山区的手机信号时断时续，电话总是打到一半就断了。
Shānqū de shǒujī xìnhào shí duàn shí xù, diànhuà zǒngshì dǎ dào yíbàn jiù duàn le.In the mountain area the phone signal cuts in and out, and calls always drop halfway.
列车钻进隧道，车厢里的灯忽明忽暗。
Lièchē zuān jìn suìdào, chēxiāng li de dēng hū míng hū àn.The train plunges into the tunnel and the lights in the carriage flicker.
她紧张得心跳忽快忽慢，手心全是汗。
Tā jǐnzhāng de xīntiào hū kuài hū màn, shǒuxīn quán shì hàn.She was so tense that her heart raced and slowed by turns, and her palms were soaked with sweat.
Common mistakes
✗ 这个房间又冷又热 → ✓ 这个房间忽冷忽热 (又……又…… joins two qualities at once, 忽……忽…… says they alternate).
✗ 忽好忽很坏 → ✓ 忽好忽坏 (both slots stay bare, one syllable and parallel).
✗ 他忽然好忽然坏 → ✓ 他时好时坏 (the mould takes 忽 or 时 alone, not 忽然).