Interrogative CPs(21) CP
C'
C IP
was it my fault
It makes perfect sense that wh-questions and yes–no questions involve the
complementiser system as complementisers contribute the semantics of force to the
sentence, but there is independent evidence for the validity of these analyses. To start,
that inverted auxiliaries occupy the C position is supported by the observation that
inverted auxiliaries and complementisers are in complementary distribution. The best
evidence for this involves not interrogative clauses, though it is also true that we never
get inverted auxiliaries and complementisers together in an interrogative, but
conditional clauses. In English there are two types of conditional clause, one formed
with an if complementiser and one formed with an inverted auxiliary:
(22) a [if he’s a government minister] then I’m the Queen of Sheba
b [had I known about your allergy] I wouldn’t have sent flowers
That if conditionals do not involve auxiliary inversion demonstrates that the
complementiser and the inverted auxiliary are in complementary distribution and
therefore occupy the same position:
(23) a [if had I known royalty was visiting] I would have combed my hair
b [had if I been told the deadline] I would have typed the report yesterday
Moreover, with many wh-questions we also get auxiliary inversion:
(24) a what will you do?
b when should we meet?
c who did you talk to?
As we can see the wh-element precedes the inverted auxiliary, supporting the
assumption that the wh-element is in the specifier of CP and the inverted auxiliary is in
the head position:
(25) CP
DP C'
what C IP
should they do
Having established the structure of these interrogatives, let us spend some time
discussing the processes involved in their formation.