内(うち)にWhile, During

When うち, (sometimes seen using the kanji 内(うち), or 裡(うち)) is paired with the case marking particle に, it expresses a period/space 'within' which something else will/should happen/exist. It quite often (but not always) carries the nuance of (A) being the 'best', or the 'optimal' time/space within which (B) could occur/exist. In English, うちに can simply be translated as 'while', or 'during'.

うちに may be paired with the dictionary or ている form of verbs, the plain form of い-Adjectives, nouns followed by の, or な-Adjectives that are followed by な.

Unlike 間(あいだ)に, うちに expresses something that can happen/exist anywhere 'within' the span of (A), and is therefore non-specific as to where (B) is located.

Fun Fact

The kanji 裡(うち) refers literally to the inner walls/lining of something, and anything that is enclosed within that space. This can be thought of as where the 'non-specificity' of うち comes from, in regard to where within that space something is.