Is it correct

Nini78Is that sentence correct?

Yes.

Nini78What is the type of the clause ?

Noun.

I am concerned about when he will arrive.
I am concerned about my sister. (My sister is a noun phrase.)

But it is clause not phrase because of "when"

Nini78But it is clause not phrase because of "when"

Correct.

It can't be an adverb clause because the when clause doesn't indicate when the rest of the sentence occurs. You can't place the when clause at the beginning and make sense of the sentence.

When he will arrive, I am concerned about. ???

Besides,

Nini78But it is a clause not a phrase because of "when"

"when he will arrive" is a clause because it has a verb (will arrive) and a subject (he).
It is a noun clause because a noun can substitute in its place in the sentence. See my last post.

"About" is a preposition.
"About my sister" is a prepositional phrase. "My sister" is t