Subject complement

In grammar, a subject complement (also called a predicative complement) is a predicative expression that follows a linking verb (copula) and that complements 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 copulas, of which be is the most common. Since copulas 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 bold in the following examples:

Here, was is a linking verb (an inflected form of be) that equates the predicate nominative phrase a tranquil pool, with the head noun, pool, to the subject, the lake (with head noun lake).

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:

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.

...
Wikipedia