day by day; day after day
day by day意为“一天一天地”，“逐日”，表示事情的逐渐变化过程。该短语只能作状语。如：
It's getting colder day by day．天气一天天冷了起来。
The boy is getting better day by day．那孩子一天天好了起来。
day after day意为“日复一日”、“一天又一天”，表示一个重复（周而复始或循环重复）的动作或事件。该短语可作主语和状语。如：
Day after day went by, and still no message arrived．日子一天天过去，仍然杳无音讯。
I have to do the work day after day．我得天天做这项工作。
