1 (used in nouns and adjectives 用于构成名词及形容词) below; less than 以下；少于: sub-zero temperatures 零下气温a subtropical (= almost tropical) climate 亚热带气候a sub lieutenant 陆军少尉

2 (used in nouns and adjectives 用于构成名词及形容词) under 底下: subway 地下铁路submarine 潜艇

3 (used in verbs and nouns 用于构成动词及名词) making a smaller part of sth 分得更小: subdivide 再分subset （组成大套的）小套