1.when既可以是并列连词,意思为：“在这时突然”；又可以是从属连词,意思是：“当.时”.如
I was watching TV yesterday when Suddenly the bell rang.昨天我正在看电视这时突然门铃响了.(并列连词）
He was doing his work when I saw him.我见到他时,他在工作.（从属连词）
2.while 也是如此,既可以是从属连词,又可以做并列连词.如
While I was writing an article,he came in.正当我写一篇文章时,他进来了.（从属连词）
I like English while he loves Chinese.我喜爱英语,而他喜欢汉语.（并列连词）
