I can only speak from an English perspective. Be seems to me to be a transitive verb, when joining a subject and an object, yet it is described as a copula.
What I mean is
The bullseye is the target
Looks to me like subject, verb, object.
Also
Where is the hospital?
Here I see where as the subject.
I don't understand why it is not.
First it is important to realise that linguistic labels do not really "exist" objectively: they are just words that can be more or less convenient.
Now let's see how things would work out if we assumed that a copula was a kind of transitive verb, and its secondary complement a direct object (the subject being the primary complement of the verb). The following are a few of the properties that direct objects are normally expected to possess—and a transitive verb is defined as having a direct object:
A direct object must always be in the accusative/objective case (that man was I v. *that man fears I — the fact that the nominative/subjective case "I" is possible with copulae shows how they are to many people different from transitive verbs; an object can never be in the nominative case. The number of people who would use "I" in certain registers prevent us from simply regarding it as an object in all respects.).
It allows for the transitive verb to be passivized, and it then becomes the subject if the meaning of the sentence is to remain intact (the cat likes me => I am liked by the cat, but she remained a cat => *a cat was remained by her ).
Only a phrase that acts like a (substantive) noun—a "noun phrase"—can be a direct object. Normally, this includes only words that can have an article without changing their meaning, unlike common adjectives (the chair is expensive v. *the chair costs expensive ).
It can be seen that copulae + subject complements fail at least one of these tests, which is why they are normally not considered transitive verbs + direct objects.
So what are the commonly used criteria for calling a verb a copula?
A. It must be capable of taking an adjective as its secondary complement in a simple sentence, like I am angry, and the adjective cannot be replaced with an adverb without change of meaning. A complement is a word or phrase without which the predicate/sentence is incomplete.
B. The adjective must assign a property to the subject. This is a semantic criterion (having to do with meaning), not a morpho-syntactic one (form). The apple is red; the number turned out larger than expected.
C. If a noun is used as the secondary complement, it must be used to identify the subject (another semantic criterion). I am a man; the caterpillar became a butterfly.
D. It cannot take an object. Note that the same verb can be a copula in one sentence and a transitive verb in another, if it is used with a different meaning / predicate frame.
E. It cannot be passivized—as transitive verbs can—such that the subject becomes a "by" prepositional phrase and the object the new subject, and that without change of meaning. It excludes examples like this: Do they prefer the small or the large car? — They prefer small. => Small is preferred by them (this should not be possible with copulae).
F. If it can take a personal pronoun as a secondary complement, a copula must be capable of taking it in the nominative case in certain contexts, i.e. for the significant minority who choose to do so, especially in writing. In other words: if absolutely everyone would always use the accusative, it's not a copula.
Normally a copula should fulfil all these criteria. I think it is safe to say that this is quite a reliable test to exclude transitive verbs.
You may have noticed that not all of these criteria are easy to check beyond doubt, especially the semantic criteria. Take this example:
I walked fast.
Here the fact that fast can be both adjective and adverb obscures criterion A (can take adjective).
Criterion D (has no object) is a bit circular, so perhaps that should be removed from the list all together and merged with criterion E (passivize): the latter will serve to prove that something is or is not an object in any case. If I use "a mile" as secondary complement, the sentence can be passivized, so perhaps it is not a copula; however, one could argue that the verb is used with a different meaning there, so that D/E do not apply at all, if we accept that walk cannot take a noun as a secondary complement.
Criterion F (nominative pronoun) does not apply at all, because one cannot walk a person.
This leaves criterion B (adjective property) to do the job, because C (noun identity) only works if the verb can take a noun as a secondary complement at all, which was shown to be dubious above. As to B—does "fast" assign a property to the subject? One could say it does, since "I" am "fast" when I'm walking fast. However, this should also be the case with other adjectives, then: *I walked rapid. This proves that "fast" was not an adjective in the original example, which had the same construction and the same meaning.
So all criteria together are a fairly tight test; but it must be noted that the boundary between copulae and non-copulae is not without weak spots. There are bound to be dubious cases. Even so, this is a useful distinction in many situations.
Why is "copula" a useful category? First of all, because copulae generally behave differently from transitive verbs and verbs that take adverbs as secondary complements.
Secondly, because subject complements used to be clearly marked as such by being in the nominative case when the precursors to modern English still used cases consistently; this makes it easier to maintain a consistent set of labels for the same construction through the ages. It is the same construction, because that man was I has not changed essentially between then and now: it is rather that the case endings of nouns were dropped gradually. So we would have to take an arbitrary point in time at which we suddenly start using a different label for this construction. In addition, the gender of the subject could be used to identify subject complements too, before endings were dropped. An adjective would agree with the gender of the subject if used as a subject complement.
Thirdly, many foreign languages still have clearly marked cases or genders, like German, in which subject complements are easier to identify than in English. The construction and the criteria I set out above are the same. Should we then call the same construction by different names in different languages? That is possible, but it obscures interlingual connections, and I would prefer to do that only if we have a very pressing reason to do so.
Fourthly, people have been using this label for ages: why change it now, and bother them to learn yet another term without a strong reason?
Perhaps you mean the difference between a copula and a non-copular verb?
The main difference is that while the copula is used as a linking verb, which defines the subject, a non-copular verb defines the action.
For example:
This is a rather clear concept for someone who studies Languages or Linguistics, but I understand it might be harder for someone who is not so into this subject, so I'll try to explain more in depth what it means to define the subject or the action.
Let me make an example in Italian: "Giovanni è felice." It means "John is happy".
The verb "è" in this sentence is not about something that Giovanni is doing, but "è felice" altogether is defining what Giovanni is (in general or in that particular moment, not really important). Note that "è felice" in the analysis is considered as one thing. In Italian it's called "predicato nominale", I'm not sure about the English name.
Defining the subject
Copular verbs don't express actions, like I said. When you state that "someone is something", that someone is not doing anything. There is no action being "made". Rather there is a feature being assigned to a subject.
Taking the example above, when we say that "John is a doctor", John is not **doctoring*. "Doctor" is what John is, not what John is doing.
Defining the action
Instead, when we say that "John ate an apple", we do have an action being defined. "Apple" is not what John is, but rather "what" is sustaining the action that is being performed by John.
On a final note, not only the verb to be can be a copula. Also verbs like to seem, or to appear can be copulae (they can be, i.e. they can be non-copular too). Check this list on Wikipedia about English copulae.
Since you mentioned "Transitivity", a transitive verb is a verb where the action is being performed by the subject and that is being sustained by an object. Read more about it on the wikipedia page for Transitivity, but to explain it here:
Transitivity is closely related to Valency which is about the possibility for some verbs to sustain other arguments other than the Direct Object, as in "John gave Mary a pen". You can see the link above or this answer I wrote on EL&U.
I'd like to add a note on a very important distinction that hasn't been covered yet.
- I am a student. (a copula)
- I became a carpenter. (a semi-copula)
- Sheila seems ill. (a pseudo-copula)
A copula is semantically empty in the sense that it doesn't "make an independent contribution to the sense of the sentence" (Hengeveld 1992: 32); that is why it can be omitted in many languages.
A semi-copula adds "a flavor" to the sentence (usually aspectual, e.g. ingressive "become"), and, when left out, it changes the meaning of the sentence:
I was a carpenter. vs. I became a carpenter.
A pseudo-copula functions as a "complement-taking" predicate:
- Sheila seems ill. - Sheila seems to be ill.
- Sheila became ill. - *Sheila became to be ill.
NB: some of the examples are taken from Hengeveld's 1992 paper. In American English, sick would sound better in the examples above.
The noun phrase which follows a transitive verb indicates a distinct participant in the state-fo-affairs indicated by the clause.
The noun phrase which follows a copula verb does not indicate a distinct participant.
In "Sam hits a doctor", there are two participants: Sam, and the doctor.
In "Sam is a doctor", there are not two separate participants. There is just one participant, Sam, and "a doctor" is a noun phrase that tells us something more about Sam.
So a noun phrase after a copula is not a direct object, because it is not a separate participant. Instead we typically refer to the noun phrase after the copula as a subject complement. (Subject complements can also be adjective or preposition phrases, incidentally.)
That's the difference between a copula and a transitive verb: a copula takes a subject and a subject complement, a transitive verb takes a subject and a direct object.
One way to test this is that transitive clauses can be passivised but clauses with a copula cannot. So, we could say "A doctor is hit by Sam" but we cannot say "A doctor is been by Sam".
In English the distinction is hardly relevant, since it doesn’t have cases except for the pronouns, but with regard to languages that do have cases it’s more relevant.
Actually, in my opinion there’s hardly any difference: the difference in languages that do have cases (Latin, Greek, Russian) is the case the verb requires with it.
Let me give examples from Russian and Ancient Greek
εἰμί +nominative case - ἐκεῖνος ὁ ἀνὴρ φιλόσοφός(nom) ἐστιν ἔδοξος(nom) = that man is a famous philosopher
ἐπιλανθάνομαι +genitive case - ἀεὶ τῶν βιβλίων(gen) ἐπιλανθάνεται = he always forgets his books
βοηθέω +dative case - ὁ υἱὸς τῷ γείτονι(dat) βοηθεῖ = the son is helping the neighbour
κολάζω +accusative case – τὸν κλέπτην(acc) βαρέως κολάζουσιν = they’re punishing the thief severly
Russian:
быть +nominative case: он -()- знаменитый философ(nom) = he is a famous philosopher
боятся +genitive case: они боятся царя(gen) - they are afraid of the czar
помогать +dative case сын помогает соседу(dat) - the son is helping the neighbour
видеть +accusative case: я тебя(acc) вижу - I see you
пользоваться +instrumental case: он пользуется влиянием(inst) - he is using his influence.
The only thing which distinguishes transitive verbs in English (or verbs that require the accusative in Russian, Greek and Latin) from intransitive verbs is transitive verbs can be put in their passive voice. They fear the czar – the czar is feared Ὁ κλέπτης(nom) κολάζεται – the thief is (being) punished Видятся дома(nom) – the houses are (being) seen
