這一對詞組的含義和使用場合並不相同。as fast as有兩個意思：（1）和……一樣快；（2）和……一樣堅固（緊密）。例：
I run as fast as you.
我能和你跑得一樣快。
All the windows are as fast as the door.
所有的窗戶都和那扇門閉得一樣緊。
在第一句中，fast說明run，是副詞；在第二句裏，fast說明windows，是形容詞。
as soon as的意思是：“一……就……”，它的同義短語有no soohan…；hardly（or scarcely）… when（or before）；the moment（or minute）….
As soon as the monkey saw the tiger,it ran as fast as it could.
猴子一看見老虎就馬上飛快地逃走。
I opehe door as soon as I heard the knock.
一聽見敲門聲我就開門。
應當在此指出，在以as soon as引導的從句中，謂語動詞一般都用簡單過去時而不用過去完成時。例如：
As soon as I heard the knock,I opehe door.
或用一般現在時而不用將來完成時。例如：
As soon as you arrive in Hong Kong,I''lle to e you.
還有一點要注意的是，as soon as引導的從句裏是不能用進行時態的。例如我們不能說：
I opehe door as soon as he was knog.
而應改作：
I opehe door as soon as he knocked.
請注意：在個別場合下，as soon as與so soon as同義，但so soon as隻能用在否定句中，而as soon as則不受此限製。例如：
He did not arrive sobras soon as we had expected.
She pleted her assigs as soon as she had intended.