うちに

1. 1Do something while a condition still holds, before it changes.Ex. 1
2. 2As an activity continues, a change gradually develops.

Neutral in register, うちに is common in suggestions and polite requests where timing matters, such as returning home while it is light or eating food while it is warm. It can also describe a change that develops during an ongoing activity; this does not mean that every use with 動詞ている形 describes an unplanned change.

• 〜ないうちにN3「〜ないうちに」 is simply 「うちに」 attached to a verb's ない form, and it keeps both meanings: doing something while you still can, and a change happening without your noticing, as in 「知らないうちに」. When the point is to act in time, the two can be rephrased into each other: 「明るいうちに」 ≈ 「暗くならないうちに」.
• 間にN4「間に」 just objectively marks off a time span with a clear start and end, and the main clause describes something that happens at some point within it or is deliberately completed within it, without stressing that a state is about to change. In situations like 'do it while it's hot,' both patterns work, but 「うちに」 carries a stronger sense of seizing the right moment.
• 中N3「中」 is a suffix that attaches only to nouns to form compounds. It objectively expresses an entire range, an action in progress, or being within a set period, but lacks the 「うちに」 nuance of acting before a state changes or of a change happening unnoticed; verbs and adjectives cannot attach to 「中」 directly.
• 最中にN3「最中に」 highlights being interrupted right in the middle of doing something, and the second clause is typically an unexpected event; it has no 'while you still can' or 'unnoticed change' meaning. It attaches only to 「noun + の」 and the 「ている」 form, and the two patterns generally cannot be swapped for each other.
• ときN5「とき」 merely states the point in time or period when something happens, with neither the urgency of acting before a state changes nor the nuance of an unnoticed change. Swapping 「ないうちに」 for 「ないとき」 leaves only 'when (something) does not happen,' so the meaning changes.

〜ないうちに specifies that something has not yet happened, while 間に marks an interval without necessarily emphasizing a condition that may end. 中 refers to an ongoing activity or period, 最中に places something right in the middle of an activity, and とき marks a time without the same emphasis on opportunity.