~うちに
Meanwhile; while... (before change)
Structure
N/A conjugate into attributive (noun-modifying) form + うちに, Vない・Vている + うちに
Explanation
Expresses doing something before a state changes; meaning 'take advantage while still...'.
Examples
1暗くならないうちに(=暗くなるまえに/明るいうちに)山を下りたい。
I want to get down the mountain before it gets dark.
2祖父は元気なうちにいろいろな所へ旅行したいと言っている。
My grandfather says he wants to travel to many places while he's still healthy.
3あしたは母の日だ。母が寝ているうちに朝ごはんを作って驚かせよう。
Tomorrow is Mother's Day. While my mother is asleep, I'll make breakfast to surprise her.
410代のうちに将来の目標を決めたいと思っている。
I want to decide my future goals while I'm still a teenager.
5「大事なことは忘れないうちにメモしておいたほうがいいですよ」
You should write down important things before you forget them.
~うちに
While... (in the process)
Structure
Vない・Vている + うちに
Explanation
Expresses an event that occurs unintentionally while doing A; often an unwanted or uncontrollable result.
Examples
1テレビを見ているうちに寝てしまい、試験勉強ができなかった。
I fell asleep while watching TV and couldn't study for the exam.
2切符を買っているうちに電車が出てしまった。
By the time I bought the ticket, the train had already left.
3しばらく合わないうちに、その子はずいぶん大きくなっていた。
I hadn't seen the child for a while, and he had grown a lot.
Note
1) Easily confuse two uses of ~うちに: 'while/while still unchanged' and 'while... then an unexpected event occurs'.2) Check verb form: Vない/Vている+うちに = 'while...' or 'before a state change'.3) The following clause determines the meaning: 'do X before it's too late/before change' = take advantage; 'while... then...' with unintended result = the second meaning.