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。