complementizer

• complementiser

complementize +‎ -er

complementizer (plural complementizers)

1. (linguistics) A subordinating conjunction that can convert a clause into a complement clause, i.e. one that completes a grammatical construction in the predicate and that describes or is identified with the subject or object.
   • "That" is a complementizer in the sentence "Mary believes that it is raining".
   • 1988, Andrew Radford, chapter 6, in Transformational grammar: a first course, Cambridge, UK: Cambridge University Press, page 299:

     However, what is of particular interest to us here is the fact that the presence of the Complementiser whether in (37) (b) above excludes the possibility of having an inverted Auxiliary in pre-subject position, as we see from the ungrammaticality of (38) below:
     (38)      *John wondered whether would he get a degree
     Why should this be? The answer we propose here is that this is because both Complementisers and inverted Auxiliaries occupy the pre-subject C position: hence, when (as in (38) above) the C position is already filled by the Complementiser whether, inversion of the Auxiliary is not possible, on the assumption that once a given position is filled by one constituent, it cannot be filled by another (i.e. we can only have one constituent occupying any given position).