Noun + 内
内 (nèi) is a preposition or localizer meaning 'inside,' 'within,' or 'inner.' It indicates that something is contained within a boundary, whether spatial or temporal. It is commonly attached to nouns, as in 国内 (domestic) or 范围内 (within the scope).
请在规定时间内完成报名。
Qǐng zài guīdìng shíjiān nèi wánchéng bàomíng.
Please complete your registration within the designated time.
国内的旅游业恢复得很快。
Guónèi de lǚyóuyè huīfù de hěn kuài.
The domestic tourism industry has recovered very quickly.
三天内必须交出报告。
sān tiān nèi bìxū jiāo chū bàogào.
The report must be handed in within three days.