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(For the commonly confused homophones, see compliment - complement)
In grammar, a complement is that which completes something. This is most usually used of a sentence. For some sentences, Subject + Verb is enough ("John runs"). For others, we instinctively feel that something more is required ("John built" -- we want to ask "what? (did John build?)"). Traditionally, the only functions called Complements were elements which may more precisely be called copular Complements. Many modern grammarians class the elements traditionally called Objects as forms of Complement. They talk of the Complement Object Direct (COd) rather than simply a Direct Object, and Complement Object Indirect (COi) rather than Indirect Object. The traditional terms are both quicker to write and clearer to use for all but the deepest students of grammar.
The copular Complement (in other words, the element traditionally called simply a Complement) follows the verb 'to be' (this verb is called the copula), and similar verbs like 'to become'. There are three forms of this complement.
- A nominal complement takes the form of a noun or noun phrase. In "My name is Peter", the Subject is the phrase 'My name'; the Verb is the copula 'is'; and that which completes the sense is the noun 'Peter'. In "It was Thursday", the Subject is the pronoun 'It'; the Verb is the past tense of the copula, 'was'; and the noun 'Thursday' completes the sentence. In "It has been a long time", the Subject is again 'It'; the verb is still the copula, but now in the present perfect tense; and the Complement is the noun phrase 'a long time'.
- An adjectival complement takes the form of an adjective. For example, in the sentence "She is Scottish", 'Scottish' is the Complement of 'is'. In "The weather was lovely", the Complement is the adjective 'lovely'.
- An adverbial complement - not surprisingly - takes the form of an adverbial, either an adverb (a single word), an adverbial phrase (a group of words), or an Adverbial Clause (a group of words containing a verb). "It is here" and "It was yesterday" are examples of the first; "It is on the sofa", and "It will be in a few days" are examples of the second; and "It is where you left it" and "That's how you like it" are examples of the third.