在定语从句中，when可作关系副词：当主句中的先行词是表示时间意义的名词且从句是完整的句子时，定语从句要用关系副词引导when引导。它作定语从句的时间状语。
例如：
1.I willll never forget the day when I joined the army.
我永远不会忘记我参军日子。
2.the days when we used foreign oil are gone.
我们用洋油的日子一去不复返了。
3.He joined the army in that year when his brother went to college.
他在他弟弟上大学的那一年参军了。
when作关系副词，引导定语从句.修饰that year.
