at once,right now,right away没有本质上区别,都表示现在马上意思.Right now只能用在现在时.比如 Finish it right now!现在就做完它!
At once 和right away可用在过去时,I did it at once/right away.我当时立马做了某事.
At the moment表示现在,目前,仅用于现在时.At the moment,we are not clear about what's happening inside the hold up bank.目前我们不知道在被抢的银行中发生些什么.