Predicative

There are two ways of using the infinitive as an adjective, 1. attributive, 2. predicative. Naturally 2. is a subject complement.

in predicative position

A predicative in Russian is an uninflected word that regularly constitutes a complete utterance when standing alone, i.e., when preceded and followed by silence.

Traditional grammars usually differentiate between neuter short adjectives and predicatives based on whether there is an overt nominative subject; hence курение вредно ‘smoking (noun) is harmful’ is viewed as a clause with nominative subject, zero copula and neuter short form adjective that agrees with the subject in gender, whereas курить вредно (same meaning, literally ‘to smoke is harmful’) is often viewed as an impersonal clause with predicative.