～うちに
while (still)
～うちに means “while”, but with a special flavor: the current condition will not last, so do the action before it changes. It attaches to い-adjectives, verbs in the ている or ない-form, and nouns with の.
Compare it with ～間に（あいだに）, which neutrally places an event inside a time span, ～うちに adds urgency since the window of opportunity is closing.
1. Do it while the state lasts
With an adjective or ている-verb, ～うちに says: act now, because soon the state will be gone.
熱いうちに食べてください。
“Please eat it while it's hot.”
- 熱い（あつい）= hot
- うちに = while (it still is)
- 食べてください（たべてください）= please eat
2. ～ないうちに: before something happens
With a ない-form verb, it means “while ... has not yet happened”, i.e. “before” the change occurs.
暗くならないうちに帰りましょう。
“Let's go home before it gets dark.”
- 暗く（くらく）= dark (adverbial form of 暗い)
- ならない = does not become
- うちに = while (still not)
- 帰りましょう（かえりましょう）= let's go home
How to form it
い-adjective / Verb ている / Verb ない-form / Noun + の + うちに
- 熱い（あつい）= hot → 熱いうちに（あついうちに）= while it's hot
- 忘れる（わすれる）= to forget → 忘れないうちに（わすれないうちに）= before forgetting
- 明るい（あかるい）= bright → 明るいうちに（あかるいうちに）= while it's light out
Key idea
～うちに = “while you still can”. The condition is temporary, so the sentence urges doing the action inside that shrinking window.
Other N3 grammar
Updated .