complementizer

A complementizer is a word or morpheme that introduces a complement clause, the subordinate clause that completes the meaning of a verb, adjective, or noun. Its main function is to mark a content clause as an argument or modifier of the main clause. In English, common complementizers include that, whether, and if; that is often optional in declarative clauses. Nonfinite complements use infinitival markers such as to in I want to leave, though analyses vary on whether to treat to as a complementizer.

In syntax, the complementizer is typically analyzed as the head of a complementizer phrase (CP) that dominates the embedded clause. The complementizer can encode finiteness (finite vs nonfinite), mood, and sometimes aspect or negation scope. Its presence or absence can influence word order and information structure, and it interacts with movement operations in more elaborate theories of sentence structure.

Across languages, complementizers are widespread and often distinguish different types of subordinate clauses. Romance languages commonly use a form of que (that) to introduce declarative complements, for example Yo creo que él viene (I believe that he is coming). German uses dass (that) in many subordinate clauses, as in Ich glaube, dass er kommt. Some languages mark interrogative or conditional content with distinct complementizers or rely on case particles, word order, or intonation to signal the clause type.

In usage, complementizers help convey reported speech, thought, intention, or attitude. They are a central element in theories of clause structure, helping to distinguish main clauses from embedded, content-bearing clauses.