George is as efficient a worker as Jack.（强调名词）

=George is so efficient a worker as Jack.(强调名词）

=George is a worker (who is) as efficient as Jack.（强调形容词）

=George is a worker as efficient as Jack.(强调形容词）

不定冠词用于固定结构quite/half/rather/many/what/such a (an)+名词或so/too/how+形容词+a(an)+名词中，表示程度、数量或感叹等.

A:quite/half/rather/many/what/such a (an)+名词

It takes half an hour to get there. 到那里需要半小时。
He's quite a famous artist. 他是个很有名的艺术家。
Many a man would welcome such an opportunity.

许多人会很高兴有这样一个机会。
What a fool he is! 他多傻啊！
It's too difficult a problem for him.对他来说，这是一个太难的问题。

I have a number of sponsored students.
Mary has a lot of money.

Tom is rather a fool.
I think Chinese is quite a useful language.
Many a student has asked that question.
The visitor left half an hour ago.
What a fine day it is!
How can you say such a thing?

B:so/too/how+形容词+a(an)+名词

We have not had so hot a day before.
Susan is as clever a girl as Anna.
That is too difficult a book for beginners.
No one knew how serious a problem it was until later.