a week before 不等于 the week before.
a week before中,中心词为before,a week是修饰语,即7x24小时以前。而 the week before中,中心词为week, before为修饰语,意为,前一周,上周,这周中的某一天或整个一周。例如下句:
He'd been driving Kim to school every day the week before he left.
他离开前那一周每天开车送Kim上学。这个句子中的the week就不能改为a week.