(1) 连词immediately / directly / instantly和词组the moment, the minute意为“一……就……”,相当于as soon as.如：
The thief ran away the moment / directly he saw the policeman.
[注意]
a. immediately / directly / instantly也是副词,在句中作状语.如：
I didn’t immediately realize how serious the situation was.
b. 介词on / upon +名词 / V-ing也表示“一……就……”.如：
Yao Ming was surrounded by the media on his arrival at the airport.
On seeing the painting by Van Gogh, I fell in love with it at once.
区别不大,只是有各自的固定搭配