When he had finished his lunch he asked for a glass of water
吃完中饭后他要了一杯水
He had finished his lunch when he asked for a glass of water
他要一杯水的时候他已经吃完了中饭
两句表达的意思大差不差,都是吃完中饭和要水两个事情,但是重心和强调的内容不一样,前一句重心是要水（吃完中饭是做状语起修饰作用）,后一句则相反