不下雨則矣，一下一定是傾盆大雨。

It never rains but it pours.

Source: Tatoeba

Sentence Composition

1 不下 bùxià to be not less than (a certain quantity, amount etc)
=2 雨 yǔ rain

classifiers: 陣, 場
=2 雨 yù
  1. (literary) to rain
  2. (of rain, snow etc) to fall
  3. to precipitate
  4. to wet
3 則 zé
  1. (literary) (conjunction used to express contrast with a previous clause) but; then
  2. (bound form) standard; norm
  3. (bound form) principle
  4. (literary) to imitate; to follow
  5. classifier for written items
4 矣 yǐ classical final particle, similar to modern 了 le
5 一下 yīxià
  1. (after a verb) a bit; a little (indicating brief duration, or softening the tone, or suggesting giving sth a try)
  2. all at once; suddenly
6 一定 yīdìng
  1. surely
  2. certainly
  3. necessarily
  4. fixed
  5. a certain (extent etc)
  6. given
  7. particular
  8. must
=7 是 shì
  1. to be (followed by substantives only)
  2. correct; right; true
  3. (respectful acknowledgement of a command) very well
  4. (adverb for emphatic assertion)
=7 是 shì variant of 是 shì
8 傾盆大雨 qīngpéndàyǔ (idiom) a downpour; rain bucketing down