fast比较级和最高级分别为faster和fastest。fast可作形容词、副词、动词和名词，基本含义有“快速的”、“立即发生的”、“头脑灵活的”、“不久”、“牢固地”、“节食”、“斋戒期”。

1、作为形容词时，意为快速的;快的;迅速的;敏捷的;迅速发生的;立即发生的;动作迅速的;头脑灵活的。

例句：I'm early─my watch must be fast.

我早了——我的表肯定快了。

He made the boat fast .

他把船系牢了。

a fast response time

迅速的反应时间

That clock's an hour fast.

那个钟快了一个小时。

2、作为副词时，意为快速;快;迅速;不久;立即;牢固地;完全地。

例句：Don't drive so fast!

别把车开得这么快！

Her heart beat faster.

她的心跳加快。

The police said that they had reacted as fast as they could.

警方说他们已尽快作出了反应。

Within a few minutes she was fast asleep .

几分钟后她就沉睡了。

She climbed the staircase cautiously, holding fast to the rail...

她紧紧地抓着栏杆，小心翼翼地爬楼梯。

3、作为不及物动词时，意为节食;禁食;斋戒。

例句：Muslims fast during Ramadan.

伊斯兰教徒在斋月期间斋戒。

I fasted for a day and half and asked God to help me.

我斋戒了一天半，祈求上帝伸出援手。

4、作为名词时，意为禁食期;斋戒期。

例句：Fasts were frequent and universally practised, river fish played an important part in the national life. 

各种禁食期是经常而普遍实行的，那时候河鱼在国民生活中发挥着重要作用