则, 乃 and 即 — three classical connectives

規則 则 = whereas — it SETS TWO CLAUSES AGAINST each other: 前者简单，后者则复杂得多。 It follows the subject.
则 also does the work of 就 in written conditionals: 有则改之，无则加勉。
乃 = is precisely — a formal affirmation: 此乃关键所在。
即 = that is to say — equating two names: 汉语即中文。 It also means at once: 即刻, 立即。
All three are written; 则 is much the commonest.