There are no hard and fast rules about this.
這事沒有什麼硬性的規定。
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麼快。
The water was rising fast.
水迅猛上漲。
He's going too fast.
他走得太快。