interrogativity in English

A polar question in English is built by putting the finite auxiliary in front of the subject. A declarative clause and its corresponding question differ in the order of two constituents and in nothing else, which makes the marking structural and unmissable. Where the clause has no auxiliary to move, the language inserts the semantically empty verb do purely so that there is something to invert, and the lexical verb then reverts to its plain form. That insertion is the strongest evidence that the marking is grammatical rather than a matter of delivery: a language that marked questions by intonation alone would have no reason to conjure an auxiliary out of nothing.

A content question adds a second requirement on top of the first. The interrogative word moves to the front of the clause, leaving a gap where its argument would have been, and the inversion still applies unless the interrogative word is itself the subject. Both movements are obligatory in a main clause and both are suspended in a subordinate one, where the question is marked instead by the complementiser whether or by the interrogative word alone with no inversion.

The state is periphrastic rather than explicit because the exponent is a configuration and an inserted auxiliary rather than a form: no verb of English carries an interrogative ending, and the do that appears is a whole word borrowed for the purpose. The obligatoriness is obligatory because a main clause question that leaves the order alone is not a question but a declarative used as one, which is a distinct and separately available thing with a distinct force. This is a clear divergence from a language that forms a polar question by intonation alone with no change to the clause at all.