a piece of

例句：

1. 1.I need a piece of paper to write on.
   我需要一张纸来写字。
2. 2. She cut a piece of cake for dessert.
   她切了一小块蛋糕作为甜点。
3. 3. He gave me a piece of advice on how to improve my English.
   他给我一些建议，帮助我提高英语水平。