时而……，时而…… / 一时……一时……: now…, now…
【六42】【六43】复句 / complex sentences (并列)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
时而 A，时而 B describes two states or two actions that keep alternating, "now A, now B": 这里的天气时而晴，时而下雨. It is neutral and descriptive, typical of narration, reports and documentaries. Each half may have its own subject or a whole clause: 窗外时而传来鸟叫声，时而传来汽车的喇叭声.
一时 A，一时 B is shorter and more literary. It takes mostly adjectives and verbs, and stresses an unstable swing, above all of mood or circumstances: 他的情绪一时高兴，一时悲伤.
In speech you also hear 一会儿……一会儿…… and 有时……有时…… (hsk1-g21). But 有时 only marks frequency ("sometimes"), without the idea of a continuous swing. The pattern describes an alternation, not a completed event: no final 了, and both halves take the same word.
Examples
这里的天气时而晴，时而下雨，出门最好带把伞。
Zhèlǐ de tiānqì shíér qíng, shíér xiàyǔ, chūmén zuìhǎo dài bǎ sǎn.The weather here is now clear, now rainy, so it's best to take an umbrella.
窗外时而传来鸟叫声，时而传来汽车的喇叭声。
Chuāng wài shíér chuánlái niǎo jiào shēng, shíér chuánlái qìchē de lǎba shēng.From outside the window come now birdsong, now the sound of car horns.
山路上，我们时而走在树林里，时而走在小河边。
Shānlù shang, wǒmen shíér zǒu zài shùlín lǐ, shíér zǒu zài xiǎohé biān.On the mountain path we walked now through the woods, now beside a stream.
他的情绪一时高兴，一时悲伤，让身边的人不知道怎么办。
Tā de qíngxù yìshí gāoxìng, yìshí bēishāng, ràng shēnbiān de rén bù zhīdào zěnme bàn.His mood swings between joy and sorrow, and the people around him don't know what to do.
那几天天气一时冷，一时热，不少人感冒了。
Nà jǐ tiān tiānqì yìshí lěng, yìshí rè, bù shǎo rén gǎnmào le.Those days it was cold one moment and hot the next, and quite a few people caught colds.
刚上幼儿园的孩子一时哭，一时笑，老师要有耐心。
Gāng shàng yòuéryuán de háizi yìshí kū, yìshí xiào, lǎoshī yào yǒu nàixīn.Children just starting nursery cry one minute and laugh the next, so the teacher has to be patient.
Common mistakes
✗ 他时而笑，时而哭了。 → ✓ 他时而笑，时而哭 (the pattern describes an alternation, not a completed event: no final 了).
✗ 时而天晴，有时下雨 → ✓ 时而天晴，时而下雨 (both halves take the same word: do not mix 时而 and 有时).