About inversion

[nq:1]Dear all, I want to know whether subject-auxiliary inversion is obligatory in the following sentences. 1. I don't know what his name is. 2. I don't know what the matter is. 3. I don't know what is the matter with her.[/nq]
The third is less correct than your version below.
[nq:1]Can we say 'I don't know what is his name'[/nq]
No.
[nq:1]'I don't know what is the matter'?[/nq]

[nq:1]I want to know whether subject-auxiliary inversion is obligatory in the following sentences. 1. I don't know what his name ... is his name/what is the matter'? Is it ok to say 'I don't know what the matter is with her'?[/nq]
Usually, inversion is not called for in subordinate clauses, as in the examples you give. I would call the exception in your list, No. 3, a vivid colloquialism. It i

[nq:1]I want to know whether subject-auxiliary inversion is obligatory in the following sentences. 1. I don't know what his name ... is his name/what is the matter'? Is it ok to say 'I don't know what the matter is with her'?[/nq]
The subject here is I and the auxiliary verb is
do (or don't) so their verb order has not been
inverted. You may be asking about the subordinate clause (but

[nq:2]I want to know whether subject-auxiliary inversion is obligatory in the following sentences.[/nq]
Don Phillipson replied:
[nq:1]The subject here is I and the auxiliary verb is do (or don't) so their verb order has not been inverted. You may be asking about the subordinate clause (but it includes no auxiliary verb.)[/nq]
I would assume Ray was, in fact, talking about word order in

snip
[nq:1]I would not consider this to be correct.[/nq]
Really? In its contracted form, it's certainly idiomatic: "I don't know what's the matter with her".

Cheers, Harvey
Canada for 30 years; S England since 1982.
(for e-mail, change harvey.news to harvey.van)