lake (which has the head noun lake).

::*The lake is tranquil. - Predicative adjective

In this example, tranquil is a predicative adjective linked to the subject the lake via the copula is (another concomitant form of be).

An example in which the subject complement is a clause is:

::*That is what I think. - Predicative clause

Other languages

In some languages, adjectives are stative verbs and do not require a copula