Grammar · Chapter 29
之内, 之中, 之外: within, among, beyond
N + 之内 / 之中 / 之外
之 is a written 的, and 内, 中, 外 are 'inside, among, outside'. 三天之内 'within three days', 朋友之中 'among friends', 工作之外 'outside work'. They frame time, groups and limits. For a box or a room just use 里: 盒子里, not 盒子之中.
Examples
- 我三天之内给你回信。 Wǒ sān tiān zhīnèi gěi nǐ huíxìn. I'll write back within three days.
- 朋友之中我最小。 Péngyou zhīzhōng wǒ zuì xiǎo. I'm the youngest of my friends.
- 工作之外，他喜欢唱歌。 Gōngzuò zhīwài, tā xǐhuan chànggē. Outside work, he likes to sing.