There are no hard and fast rules about this.
这事没有什么硬性的规定。
Research indicates that eating habits are changing fast.
研究显示，饮食习惯正迅速改变。
Things are not moving as fast as we hoped.
事情的`进展不像我们希望的那么快。
The water was rising fast.
水迅猛上涨。
He's going too fast.
他走得太快。