I recently answered a question on ELL. It included a sentence similar to this:
If it's neither A nor B, I assume that it's C?
I answered the question, and noted that I think that a clause beginning "I assume..." should not end with a question mark. The OP disagreed with me, and said that it was to indicate that he would use intonation to make the clause a question- to get confirmation that his assumption was correct.
My opinion is that this particular clause can't be made into a sensible question using intonation. Is this opinion reasonable?
If so, I have some theories about why this might be.
You can't use intonation to make a subordinate clause of an if into a question
You can only use intonation to make follow-on questions, and this sentence doesn't seem to be a follow-on question.
You can only use intonation to change the main clause into a question. This clause contains two sub-clauses: "I assume" and "that it's C". While it's reasonable to question the that-subclause, it's not reasonable to question the "I assume" subclause. It's a fact that you are assuming something: the only doubt is over whether the assumption is correct.
It seems to me that, when you use intonation for a question, it affects the main clause only, so using intonation with this sentence asks a pointless question. The only way to ask the correct question (is my assumption correct) is by adding an auxiliary verb:
Should I assume that it's C?
This question was closed as a duplicate of this one. The accepted answer to that question provides a series of sentences, many of which are declarative questions, however the first is an example of colloquial ellipsis (the subject and verb are omitted, so you can't tell whether subject and verb would have been inverted), and the remainder are follow-on questions.
I suggested above that maybe declarative questions are valid for follow-on questions. For my sentence, that would work like this:
Person A: If it's neither A nor B, what should you do?
Person B: If it's neither A nor B, I assume that it's C?
This is from section 32 of the Oxford Guide to English Grammar (John Eastwood, 1994):
In informal conversation a question can sometimes have the same word order as in a statement. The question has a rising intonation.
The machine gives change? ~ No, it doesn't.
You're travelling tomorrow?~ Yes.
The car is blue?~ That's right.
The car is what colour? ~ Blue.
They went which way?~ That way.
We use this kind of question only when it follows on from what was said before.
I need a return ticket to Paddington. ~ You're travelling when? ~ Tomorrow.
Are there any other circumstances where declarative questions are permissible?