There are no hard and fast rules about this.
這事沒(méi)有什么硬性的規(guī)定。
Research indicates that eating habits are changing fast.
研究顯示，飲食習(xí)慣正迅速改變。
Things are not moving as fast as we hoped.
事情的`進(jìn)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。