In German, one has to invert questions in a sub-sentence. Not doing so feels wrong in English to me, is it allowed in English too or is that strictly illegal grammar?
"Now the question is only, how accessible is the XML structure." (not inverted)
"Now the question is only, how accessible the XML structure is." (inverted)
"I wonder, how does he do it?" (inverted)
"I wonder how he does it." (not inverted)
There are two distinct processes affecting word order in questions:
Subject-verb inversion. This shows up in yes-no questions, but only auxiliary verbs are typically inverted in modern English, so a form of “do” is added with non-auxiliaries (part of a phenomenon called “do-support”). Is the appointment at 9:30? is a yes-no question with subject-verb inversion.
Wh-fronting. Interrogative words like what, when, whom, how come at the front of their clause, even if their grammatical role is something that would normally come in a different position (like the object or complement of the verb). So we put when at the start of When is the appointment? even though an answer will use a word order like The appointment is at 9:30.
A typical non-embedded question has both wh-fronting and subject verb inversion (How are you doing?) unless the subject is a wh-word (Who is at the door?).
An embedded question shows wh-fronting, but usually does not show subject verb inversion: I asked them how they were doing or I want to know when the appointment is. Inversion (I asked them how were they doing, I want to know when is the appointment) is sometimes an acceptable alternative in some dialects, but as a rule, inversion is not preferred in this context in formal English. However, using a comma before the inverted clause often increases the acceptability in standard written English: “I want to know, when is the appointment?”
