Parts of speech

[Doughnut]

Could you help me, please?
I'm trying to understand, what part of speech the word "alien" in the following sentence is:
He is alien to me.
For the first sight we would say it's an adjective. But there is another structure: He is an alien to this work. , where "alien" is a noun. And these two structures are very similar (or even have the same meanings). So, might it be that "alien" is a noun (or a substantive adjective) in the first sentence? Could there substantivation take place? Or there should be an obligatory article in front of "alien" in this case?
Thanks in advance.

[Casiopea]

He is alien.(Adjective. It's synonymous with different)

Adjective Test: He seems alien/different. (OK)

This is new to me. (Adjective)

[Doughnut]

Thanks, Kasiopea. Of course, you are right. And I understand it. But I just wanted to "investigate" this word and this sentence. Thanks again.

[Casiopea]

Thanks, Casiopea. But I just wanted to "investigate" this word and this sentence.

You're welcome.

By the way, here's a test you can try: if the word/phrase in question renames the subject, then it's substantive; if it adds information to the subject, then it's non-substantive.

Word
He is an alien => An alien is who he is. (Noun)
He is alien => Alien is what he is. (Adjective)

Phrase
He is alien to me => Alien to me is what he is. (Adjective)
He is an alien to me => An alien is who he is. (Noun)

You can also try changing the word order:

Word order
To me, he is alien. (Adjective)
To me, he is an alien. (Noun)

He seems to me to be alien. (Adjective)
He seems to me to be an alien. (Noun)

The man is young. (Noun is Adjective)
The young are men. (Substantive adjective are Noun)

But, but, but, when we look at the semantic side of things, the substantive comes into play:

He is alien to me = He is strange. (Adjective)
He is alien to me = He is a stranger. (Noun, substantive adjective)

If 'alien to me' is synonymous with 'a stranger', then it's functioning as a noun.

[Doughnut]

Casiopea, you write:
He is alien to me = He is strange. (Adjective)
He is alien to me = He is a stranger. (Noun, substantive adjective)

So then I have 2 questions:
1. It's possible, that in examples like mine - smd is alien to smb - alien is a noun, isn't it? Even if there is no article? Why do we omit article in this case?
2. What is the difference then between these 2 phrases:
He is alien to me (= He is a stranger) and He is an alien to me?

Thank you very much for such a detailed explanation. And I am sorry for possible mistakes in spelling and shaping (?) my thoughts, as English is not my native language.

[Casiopea]

It's possible, that in examples like mine - smd is alien to smb - alien is a noun, isn't it? Even if there is no article? Why do we omit article in this case?

Yes, it's possible, but I don't know how to answer your question, sorry.

[Doughnut]

Casiopea, so there are situations when even native speakers of English can't explain some phenomena in the language, aren't there?

[Casiopea]

Not just native speakers, Hun. Linguists, too.

[Doughnut]

Casiopea, what a pity! But thank you anyway!

[Casiopea]

You're welcome.