In grammar, a subject complement is a predicative expression that follows a linking verb (copula) and that complements (completes) the subject of the sentence by either (1) renaming it or (2) describing it. In the former case, a renaming noun phrase such as a noun or pronoun is called a predicative nominal. An adjective following the copula and describing the subject is called a predicative adjective. In either case the predicative complement in effect mirrors the subject. Subject complements are used with a small class of verbs called linking verbs or copula, of which be is the most common. Since copula are stative verbs, subject complements are not affected by any action of the verb. Subject complements are typically not clause arguments, nor are they clause adjuncts.
The subject complement is in bold in the following examples:
- The lake was a tranquil pool. - Predicative nominal as subject complement
- The lake is tranquil. - Predicative adjective as subject complement
In this example tranquil is a predicative adjective linked through the verb is (another inflected form of be) to the subject the lake.
An example in which the subject complement is a dependent clause is:
- That is what my point is. - Predicative clause as subject complement
Other languages 
Some languages do not use predicative adjectives with a linking verb; instead, adjectives can become stative verbs that replace the copula. For example, in Mandarin Chinese It is red is rendered as tā hóng, which translates literally as It red. However, Mandarin retains the copula when it is followed by a predicative nominal.
Disputed pronoun forms 
While no strong arguments other than widespread acceptance are made for the use of colloquial it is me (it is him, he is taller than him, etc.) in written speech in Joseph Crayton's works, other grammarians, among whom were Baker (1770), Campbell (1776), and Lindley Murray (1795), give the reason why the first person pronoun must be I rather than me: it is a nominative that is equivalent to the subject, and as such they prove that it must always be in the nominative (subjective) case. These three partisans of the nominative case, Baker, Campbell, and Murray, were the commentators whose preachments were accepted as gospel by the schoolmasters.
This argument for it is I is based on the model of Latin, where the complement of the finite copula is always in the nominative case (and where, unlike English, nominative and accusative are distinguished morphologically in all nominal parts of speech and not just in pronouns). The situation in English may, however, also be compared with French, where the historical accusative form moi functions as a so-called disjunctive pronoun, and appears as a subject complement (c'est moi, 'it is me'). Similarly, the clitic accusative form can serve as a subject complement as well as a direct object (il l'est 'he is [that/it]', cf. il l'aime 'he loves it').
Joseph Priestley justified the colloquial usage on the grounds of good writers using it often:
All our grammarians say, that the nominative cases pronouns ought to follow the verb substantive as well as precede it; yet any familiar forms of speech, and example of some of our best writers, would lead us to make a contrary rule; or, at least, would leave us at liberty to adopt which we liked best.
Fiction writers have occasionally pointed out the colloquialisms of their characters in an authorial comment. In The Curse of the Golden Cross, for example, G. K. Chesterton writes, "'He may be me, said Father Brown, with cheerful contempt for grammar." And in The Lion, the Witch and the Wardrobe, C. S. Lewis writes, "'Come out, Mrs. Beaver. Come out, Sons and Daughters of Adam. It's all right! It isn't Her!' This was bad grammar of course, but that is how beavers talk when they are excited."
See also 
- Copula (linguistics)
- Disjunctive pronoun
- Disputed English grammar
- English personal pronouns
- Predicate (grammar)
- Predicative expression
- The Merriam-Webster Dictionary of English Usage (Merriam-Webster, 1989), pp. 566-67.
- Peter V. Jones and Keith C. Sidwell, An Independent Study Guide to Reading Latin (Cambridge University Press, 2000: ISBN 0-521-65373-8), p. 11.
- The Rudiments of English Grammar (1772), p. 104.