What Are Complementizers?

Introductory Words for Complement Clauses

complementizer
In the title of this children's book by Dr. Seuss, the word that functions as a complementizer. (Random House, 1965)

In English grammar, a complementizer is a word used to introduce a complement clause, including subordinate conjunctions, relative pronouns, and relative adverbs. For example, if functions as a complementizer in the sentence "I wonder if she will come." 

In some contexts the complementizer that can be omitted — a process known as "that complementizer deletion." For example, "I wish that I had duck feet" can also be expressed as "I wish I had duck feet." The result is called a null complementizer.

In generative grammar, complementizer is sometimes abbreviated as Comp, COMP, or C. The words "that," "if" and "to" are the most popularly utilized complementizers in the English language, though the list of complementizers is quite a bit more extensive.

Common Complementizers

Although not exhaustive, Laurel J. Brinton lays out a list of the most commonly used complementizers in the English language book "The Structure of Modern English: A Linguistic Introduction." This list includes while, since, because, although, if, when, so that, as such, before, after, until, as long as, as soon as, by the time that, once, and inasmuch as."

That, if, and to have special usage as complementizers. For that, the compliment associated with a complement type is named the that-clause and may or may not be omitted and still make sense in the context of a sentence. If can function in the exact same manner as "that" as in "I don't know if John will join us."

As Michael Noonan describes in "Complementation," the word to is used in conjunction with most infinitives wherein "neither the verbal noun nor participial complement types have complementizers in English."

Adverbial Clauses and "Wh-" Questions

Similar to the that-clause and the if-clause, the adverbial clause cannot be interrogative or imperative in conjunction with the rest of a fully formed sentence.

Adverbial clauses also start with a complementizer but can utilize a far larger variety of words and types to serve as complementizers.

Similarly, "wh-" questions always begin with a complementizer, including such words as who, whom, whose, what, which, why, when, where and how. The importance difference between these and adverbial clauses though lies in the complementizers themselves.

In "wh-" questions, the complementizers — which come in the form of "wh-" words — always serve a function in their clause. As Laurel J. Brinton puts it, If the wh-word is removed, the clause usually becomes incomplete." Also, she adds, "the form of the wh-complementizer depends upon its function." 

Take for example the wh-complementizer "why" in the sentence "Why don't we go to the movies?" The "wh-" word was determined by its intended function in the wh-question "why don't we go?" wherein it was supposed to provide an inquiry into the reason the audience doesn't want to go to the movies. Further, "don't we go to the movies" no longer gives the audience the same intended message.

Thing to Remember

It's important to remember when trying to identify and use complementizers in English writing and reading that not all of the words identified as common complementizers exclusively belong to that part of speech — words like "that," "while," and "if" serve a multiplicity of functions, ranging from nouns to adverbs, with each usage meaning something different.

Still, complementizers are nigh essential to eloquent English usage and style. Even in this article, the writer has used several complementizers to further points as well as smooth transitions between thoughts and phrases.