1. quick 往往强调动作“迅速地”发生或完成，通常是非常突然或持续时间很短的动作，含有“匆忙”的意思；也可指人的思维敏捷。例如：

He gave her a quick kiss.

He had a quick look at it.

After a quick breakfast, he hurried to school leaving his bag at home.

注：催人快做/快走等，通常用 quick。如：

Be quick!

2. fast 往往指动作速度快（侧重速度），即可做形容词，也可做副词。例如：

She ran as fast as she could.

You are reading too fast for me to follow.

A train is much faster than a bus.

注：询问速度或表示钟表走得快等，通常用 fast。如：

How fast is the fish swimming?

My watch is 5 minutes fast.

3. soon 则表示时间上很快即将发生。例如：

His father will be back to China very soon.

I will inform him of this as soon as possible.

4. rapid 稍正式些，通常指突然或急速（且连续不断）的动作。例如：

He heard a rapid burst of machine-gun fire.

He asked questions in rapid succession.

注：rapid 还有一个特殊用法，即指一种（相对的）有益的高速度。例如：

He made rapid progress in English.