NEI-4

('nei' in the fourth tone)

There is only one standard Chinese character that is spoken 'nei' Apart from that, the morpheme may be heard as a colloquial variation of the 'na' morpheme.

内 = in, inside, within

To express that something is in, inside, or within somewhere, one cannot just use 内 as a preposition. Rather, in Chinese, a specific grammatical construction is needed, which involves 在 as a preposition and 内 as a postposition.

Inscription on Chinese railway tickets:

在3日内至有效 = Zai(4) san(1) ri(4) nei(4)dao(4) you(3)xiao(4) = Valid within three days 

有效 = you(3)xiao(4) = effective, valid

效 = xiao(4) = effect