Let's begin at once,it's already late.我们马上开始吧,已经晚了.Let's begin at once这句的主语是什么啊?at once是做什么的?不要it's行吗?because i don't know.

Let's begin at once,it's already late.我们马上开始吧,已经晚了.Let's begin at once这句的主语是什么啊?at once是做什么的?不要it's行吗?because i don't know.
Let's begin at once,it's already late.
我们马上开始吧,已经晚了.
Let's begin at once这句的主语是什么啊?
at once是做什么的?
不要it's行吗?
because i don't know.

Let's begin at once,it's already late.我们马上开始吧,已经晚了.Let's begin at once这句的主语是什么啊?at once是做什么的?不要it's行吗?because i don't know.
It's already late
这里的it功能和it's raining/it's Monday等句型一样,这些都是'无主语句",it除了是代词'它'的含义,还一个功能就是补充担当无主语句的'主语'.只有语法功能,不代表其它的含义.（有点象占位符)
Let's begin at once
at once=right away.
这是祈使句,也是无主语句.
字典:
used in the normal subject position in statements about time, distance, or weather. (上面的例子)

Why! Why did you ask such a question?

at once 是指马上，立刻得意思。而Let's begin at once这句话本身是祈使句，直接翻译过来就是“让我们……”得意思。原来得句子是：let us begin at once。

真把我问到翻脸了。。。