The analysis begins some distance from where most investigations would start. It seems to me that the significance of the copula is not properly appreciated but without an understanding of the difference between a copula, a transitive verb and an intransitive verb, the syntax of the language cannot be understood. The basic semantic distinction between a state and an event is captured in the syntactic distinction between copulas, which describe a state or a change in state, and transitive and intransitive verbs, which describe events.
A copula is also sometimes called a linking verb because it links a subject to a complement. It is used to identify one thing with another, to make a classification, and to attribute properties. The concept of a copula comes from logic where a copula links a subject and a predicate.
Copulas express six basic ideas that describe a state or a change in state. Many guides to grammar will argue that only the verb to be is a grammatical copula. But if some something is something, then it can become, remain, prove to be, appear to be and be perceived to be something also.
The difference between a copula on the one hand and transitive and intransitive verbs on the other is that copulas focus on a state or a change in state whereas the other two types of verb place the focus on the event.
Copulas have a number of functions: attribution, identification and classification.
A copula requires a complement because the function of the copula is to link a subject with a predicate. Complements provide information about the subject which is the purpose of the statement and for this reason they are mandatory. They provide the answer to implicit questions about who, what or which.
The infinitive and gerund form of copula verbs may also form part of a verb chain. As such, they may also form part of the complement themselves. For example, in a statement such as we remain to be convinced the finite verb is a copula and the complement is also a copula in the infinitive form with its own complement. In this case to be convinced is the complement of remain and convinced is the complement of to be.
The distinction between a complement and an adverbial is that an adverbial provides additional information about the action described in the verb. Adverbials give the answer to implicit questions about where, when, how and why. Because they provide additional information, adverbials are always optional.
The following pair of examples demonstrates the distinction between a copula and an intransitive verb:
In the copula statement, calm is a complement that describes the state that we, the subject of the statement, are remaining in. In the intransitive statement, quickly is an adverbial which further describes the action, growing, that we are engaged in.
The same verb can function as a copula, as a transitive verb and as an intransitive verb. As the example below demonstrates, the same event can be described in different ways depending on which aspect is being bought into focus.
The use of a copula describes the change in the state, in this case the height of the plants, without being concerned with why or how this is occurring. The use of an intransitive verb, on the other hand, focuses on the event, the growing, rather than the change in state. The use of a transitive verb takes the focus to the agent responsible for the event. In the active voice the agent is identified. In the passive voice, agency is implied but the identity of the agent is secondary and may be omitted.