则: whereas, then (contrast / consequence)
连词 / conjunctions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
则 is a very versatile written connector:
1. Contrast between two subjects: "A…, whereas B…": 北方干燥，南方则潮湿. 则 goes after the second subject.
2. Consequence of a condition: 若/如果…，则… = "if…, then…".
3. In nouns: 规则、原则 (rule, principle), don't confuse.
In writing it often replaces 却 (contrast) or 就 (consequence).
Examples
老城区人口越来越少，新区则发展得很快。
Lǎo chéngqū rénkǒu yuè lái yuè shǎo, xīnqū zé fāzhǎn de hěn kuài.The old districts are losing people, whereas the new ones are growing fast.
若交通问题得不到解决，则城市的发展会受到限制。
Ruò jiāotōng wèntí dé bu dào jiějué, zé chéngshì de fāzhǎn huì shòudào xiànzhì.If traffic problems aren't solved, urban development will be held back.
有人喜欢热闹的大城市，我则更喜欢安静的小镇。
Yǒu rén xǐhuan rènao de dà chéngshì, wǒ zé gèng xǐhuan ānjìng de xiǎozhèn.Some love lively big cities; I, on the other hand, prefer quiet small towns.
Common mistakes
✗ 则 before the subject: ✗ 则南方潮湿 → ✓ 南方则潮湿.
✗ 则 in speech sounds bookish → ✓ 却 / 就 / 而.