うちに

うちに can be used after a noun, adjective, or plain non-past verb to convey the meaning "while still..." When used with a negative verb, the meaning changes to "before..." When used with na-adjectives, there must be a な placed between the two, and with nouns a の must be used.

The word 間に can be used very similarly, although it cannot be used with negative verbs. Another difference is the implied meaning - with うちに you are implying that "it is not possible, easy, or a good idea to do something after the time specified by the うちに clause." The same is not true for 間に.

熱いうちに、どうぞ召し上がって下さい。
Please eat it while it's still hot.

暑くならないうちに、犬の散歩をしてきた方がいいよ。
Before it gets hot, you'd better come take the dog for a walk.

両親は、元気なうちに外国に旅行することにした。
My parents decided to take a trip to a foreign country while they're still lively.

子どものうちに外国語を勉強すると発音がよくなるそうだ。
It seems like you can improve your pronunciation if you study a foreign language while you're still a child.