A substantive standing in the predicate, but describing or defining the subject, agrees with the subject in case and is called a predicate nominative.
A predicate nominative is often called a subject complement or an attribute.
The predicate nominative is common after IS and other copulative verbs and after certain transitive verbs in the passive voice.
1. Chemistry is a useful science.
2. Boston is the capital of Massachusetts.
3. Jefferson became President.
4. This bird is called a flamingo.
5. Mr. Hale was appointed secretary.
6. Albert has been chosen captain of the crew.
7. You are a friend upon whom I can rely.
In most of the examples, the predicate nominative has one or more modifiers. In the first sentence, science is modified by the two adjectives a and useful. In the second, capital is modified by the adjective phrase of Massachusetts, In the last, friend is modified by the adjective clause upon whom I can rely.
A noun clause may be used as a predicate nominative.
1. My plan is that the well should be dug to-morrow.
2. His intention was that you should remain here.
3. The result is that he is bankrupt.
4. Ruth’s fear was that the door might be locked.
An infinitive may be used as a predicate nominative.
1. To hear is to obey.
2. My hope was to reach the summit before dark.
3. Their plan was to undermine the tower.
4. My habit is to rise early.
The infinitive may have a complement or modifiers. In the second and third examples, it takes an object. In the fourth it is modified by an adverb.