うちに
Usage
When someone does something before a situation or state changes. 間に is used in similar situations, but it cannot be used with negatives forms of verbs. Another difference between the two phrases is that うちに always implies that it is not possible, easy or a good idea to something after the time specified by the うちに clause. However, this is not always the case with 間に. Thus, if it is not a problem for the speaker to go to Japan after finishing his Japanese class, 間に should be used. While - still -ないうちに = before (something happens)
Examples
日本にいるうちに、色々なところに旅行に行きたい。
While I go to Japan I want to travel a lot of different places.
暑いうちに、どうぞ召し上がってください。
While it is hot it gets hot, please eat.