〜うちに: act before the situation changes
Compare acting while an opportunity remains with a change that develops while a condition continues.
Use the opportunity while it lasts
スープが温かいうちに飲んでください。 asks someone to drink the soup while it is warm. Once it cools, that favorable condition is gone.
Use 学生のうちに with a noun and 元気なうちに with a na-adjective. The focus is on an opportunity before the state ends, not just a neutral interval.
Use the negative for before a change occurs
忘れないうちにメモします。 means making a note before you forget. You act while you still remember.
雨が降らないうちに帰りましょう。 suggests going back before it rains. The negative ない does not negate the following action 帰る.
Recognize change developing over time
毎日練習しているうちに、少しずつ読めるようになりました。 describes gradually becoming able to read through daily practice.
This differs from urging a single action before a deadline. Check whether the following clause gives a suggestion or plan, or the result of a developing change.
Quick check: Does 忘れないうちに書いておきます mean writing something down after forgetting it?
Show the answer and why
No. It means writing it down while you still remember, before you forget. Track the time relationship expressed by ないうちに.