So and too in pattern so/too [adj] a [n] for plurals

[Grablevskij]

In a textbook in my native language I can read a rule that an indefinite article is put before singular countable nouns determined by adjectives preceded bу so or too:

It is not so simple a problem as it seems.
It is too urgent a matter to postpone.

And a note: such phrases are impossible for plural nouns.

Well. But how to express such sense for plural nouns? Let's take the very two phrases. How to express these ideas for plural nouns?

 

[Grablevskij]

But could you tell me whethe the rule is really correct or not?

 

[jutfrank]

Yes, the rule in your textbook is correct. What makes you think it isn't?

I'd ask you to stop thinking about correct/incorrect rules and focus instead on the pattern and how it's used.