predicatives (AKA subject and object complements)

Predicatives are nominals or adjectivals which tell us more about the subject or object by means of the verb.

In the following examples the predicative is telling us more about the subject. Subject predicatives are necessary sentence elements, i.e. if they are removed well-formed sentence does NOT remain.

The bag seems heavy. (adjectival)
That man is a thief. (nominal)
In the following examples the predicative is telling us more about the object. Object predicatives are non-obligatory sentence elements, i.e. if they are removed a well-formed sentence does remain.

We painted the house yellow. (adjectival)
They elected him president. (nominal)