right now;at once; right away; immediately.

at
once,
immediately,
right
now,
right
away
这四个短语常可互换，只是说话角度上有细微区别。
1.at
once等于now,
without
delay。如：
Do
it
at
once.立刻就做。
Come
here
at
once!马上就来！
at
once
也有“at
the
same
time,together”的意思。如：
Don’t
all
speak
at
once!
不要全体同时说话！
注意：all
at
once=suddenly
2.immediately和at
once相同，但语意上侧重于“紧接着”。如：
She
answered
almost
immediately.她几乎立刻作了回答。（紧接着问，立刻作答）
He
will
be
ready
immediately.他马上就会准备好。（暗指还需几分钟）
注意：immediately还可做连词，等于as
soon
as。如：
I
told
him
immediately
he
came.他一来我就告诉他了。
3.right
now指at
this
moment,
right
away指毫不犹豫或毫不耽误的“立即”。如：
If
you
get
the
address,
let
me
know
right
away.你看到地址，请立即告诉我。
Shall
we
begin
right
now?
我们现在就开始吗？