There are no hard and fast rules about this.

　　這事沒有什么硬性的規定。

　　Research indicates that eating habits are changing fast.

　　研究顯示，飲食習慣正迅速改變。

　　Things are not moving as fast as we hoped.

　　事情的進展不像我們希望的那么快。

　　The water was rising fast.

　　水迅猛上漲。

　　He's going too fast.

　　他走得太快。