〜うちに
while (still), before (it changes)
Verb / い-adj / Noun の + うちに · Verb ない-form + うちに
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うちに means "while it's still…", doing something before a situation changes: 温かいうちに食べてください, "please eat it while it's hot"; 若いうちに旅行したい, "I want to travel while I'm young".
With a negative it means "before": 雨が降らないうちに帰ろう, "let's go home before it rains" (literally "while it's not raining yet"). 忘れないうちに, "before I forget", is a common one.
Compare 間に, which is simply "during a period". うちに carries a sense of urgency: the chance won't last.
Nouns take の: 学生のうちに, "while you're still a student".
Examples
温かいうちに食べてください。
Please eat it while it's hot.
若いうちに、いろいろな国に行きたい。
I want to travel to lots of countries while I'm young.
雨が降らないうちに、帰りましょう。
Let's go home before it starts raining.
忘れないうちに、メモしておきます。
I'll write it down before I forget.
明るいうちに、駅に着きたい。
I want to get to the station while it's still light.