suupu ga samenai uchi ni, nabe ni futa o shite kudasai.

Before the soup gets cold, please put a lid on the pot.

Breakdown of suupu ga samenai uchi ni, nabe ni futa o shite kudasai.

• が: subject particle
• スープ: soup
• ください: please
• 〜ない: negative form

Questions & Answers about suupu ga samenai uchi ni, nabe ni futa o shite kudasai.

What does うちに mean in this sentence?

Here, うちに means something like while ... is still the case or before ... changes.

So 冷めないうちに means:

• while it hasn’t cooled yet
• before it gets cold
• before it cools down

This pattern is very common:

• 忘れないうちにメモします。
  I’ll make a note before I forget.
• 熱いうちに食べてください。
  Please eat it while it’s still hot.

In your sentence, the idea is: do the action now, before the soup loses its heat.

Why is it 冷めないうちに and not 冷めるうちに?

Because うちに often follows a negative form when you mean before something happens.

• 冷めないうちに = before it cools
• 冷めるうちに would not sound natural here

The negative form sets up the idea that the current state is still true:

• the soup has not cooled yet
• so act during that remaining time

This is a very useful pattern:

• 暗くならないうちに帰ろう。
  Let’s go home before it gets dark.
• 忘れないうちにやっておく。
  I’ll do it before I forget.

So ない + うちに is a standard and natural pattern.

What is the difference between 冷めないうちに and 冷める前に?