1.Few people will pay attention to their health ( until）they get disease.
比较：Few people pay attention to their health ( before）they get disease.
2.We were chatting on the net ( when ) the light went out.
when 在这里是并列连词“其时” “就在这个时候” ,用来引出一个突发动作（后一分句）,until 无此用法
3.Three years had passed( before )i knew it
这里before 用在肯定句前表否定,when 无此用法
意为：不知不觉又是三年过去了
再如：
He left before I could say hello to him
我还没来得及同他打招呼,他就走了.