Complementizers

Complementizers are a class of words or morphemes that introduce complement clauses—the clauses that function as arguments of a verb, adjective, noun, or preposition. They mark the embedded clause as a dependent or subordinate clause and often carry information about mood, tense, or other grammatical features. In many languages, a dedicated complementizer heads the embedded clause, forming a complementizer phrase (CP) that connects the clause to the matrix predicate. Complementizers can be overt (visible) or null (zero).

English uses that, if, and whether as complementizers. I think that she left; I wonder if she left. Some varieties allow the that to be omitted: I think she left. In nonfinite clauses, infinitival to can serve as a complementizer: I want to go. Other languages have richer systems: German dass for a finite clause; ob for yes-no questions; French que; Spanish que or si with subjunctive in subordinate clauses. Some languages also have multiple complementizers encoding nuance such as evidentiality, polarity, or illocutionary force.

Cross-linguistically, complementizers interact with the syntax of the embedded clause and may trigger word order adjustments or agreement. They can be obligatory in some languages and optional or zero in others. Historically, many complementizers derive from conjunctions or relative pronouns and have evolved different uses in various language families.