Adjectives cannot be grammatical subjects
No, you cannot do that. The reason is somewhat trickier than you might imagine.
So to start with, of course you can say this:
- Spanish is an easy language for a Frenchman to learn.
Because it’s here the name of a language, Spanish takes a singular verb.
But you cannot say:
- ❌ Spanish are increasingly restless. [UNGRAMMATICAL]
That’s illegal. Since adjectives in English lack number, they cannot be the subject of a clause.
Instead you must use a plural noun and say:
- Spaniards are increasingly restless.
By using a plural noun, this of course takes a plural verb.
However, when you write:
- The Spanish are increasingly restless.
You do not have an adjective as the subject. And yet the word Spanish
there is not a noun, either.
And this is where it gets a bit tricky.
What you do have is a clause that uses a fused-head modifier
as its subject. The fusion is that the actual head of the noun phrase, some
sort of noun, has fused into the modifier, but it is still a noun phrase
nonetheless and so can be the subject of a clause. What it has fused with
is some plural noun like people, which is why it still takes plural
concord.
This is the same thing that happens with fused-head constructions like these:
- The tired are with us still.
- The poor are with us still.
- The huddled are with us still.
- The wretched are with us still.
None of those sentences has any nouns in it. You can apply intensifiers to
adjectives but not to nouns, and those can all take intensifiers without a
hitch:
- The most tired are with us still.
- The very poor are with us still.
- The rather huddled are with us still.
- The really wretched are with us still.
If those had been nouns, you could not have done that. Since you can do
that, those are not nouns. They are adjectives forming part of fused-head
constructions. The original nouns are fused and gone.
If you want to use just an adjective and have that be an entire noun
phrase, you need to have a determiner precede it to signal that
the head noun has been fused.
There are other types of fused heads than adjectives, but we can save those for some other day.