once

at once, directly, immediately, instantly, presently, right away

这组词(组)共同的意思是“立即”“马上”。它们的区别在于:
1.at once主要指动作迅速,但中间多有一段距离,且at once比immediately意味更强。例如:
I'm leaving for Rome at once.我马上去罗马。
He gave orders for the work to be started at once.他下令立即开始那项工作。
2.immediately侧重指时间“紧接着”。例如:
I came immediately after I'd eaten.吃过饭我立即就来了。
Please close your books immediately and answer these questions.请立即合上书,回答这些问题。
I left immediately when the clock struck twelve.钟刚打12点我就离去了。
3.right away经常出现在美式英语中。例如:
I'll do it right away.我立即去办。
He had written down a list of things to do right away.他已写下须立即做的事情。
Don't expect them to approve of the design right away.不能期望他们马上批准这项设计。
4.instantly也是指动作迅速,但它更强调的是同时发生,中间有很短的距离或无距离。例如:
The electrician went instantly in answer to the phone call.电工师傅一接到电话马上就去了。
He was killed instantly in a car accident.在车祸中,他当场死亡。
5.directly强调“直接地”,多与“迂回绕过”相对。例如:
Whether he sailed directly across the bay or coasted round it is uncertain.他是径直驶过海湾,还是沿海岸迂回绕过的,现在还无法确知。
6.presently强调过一会再去做某事。例如:
I will do the dishes presently, but I want to finish the story first.我过一会去洗碟子,不过我想得先看完这篇故事。

at once, soon

这两个词(组)的区别是:soon表示“不久”“很快”,强调时间很短; at once表示“马上”“立刻”,强调动作随即发生。例如:
He soon became active in the class.他在班上很快就变得活跃起来。
When I heard the knock,I opened the door of the room at once.我听到敲门声马上就把房门打开了。

once，as soon as

两者均为从属连词，引导时间状语从句，都表示主句的动作紧跟在从句的动作之后发生。
once除表时间外，还表示条件，一般译作“一旦......就......”。
as soon as强调的只是时间，译作“刚......就......”或“一......就......”。