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I am quite confused with this. Sentence is:

His dog is his best friend.

I would say that this is copula sentence, without object, and that HIS DOG is a subject. My friend is trying to persuade me that it is quite different i.e. his dog=object, his best friend=subject. Pls help&thank you

closed as off-topic by RyeɃreḁd, Ronan, user66974, tchrist, anongoodnurse May 28 '14 at 3:36

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    In your sentence, dog is the subject, is is the copula, and friend is the predicate nominative. – Theodore Broda May 27 '14 at 14:44
  • Your question pertains to basic aspects of English grammar, such as subject and predicate; I suggest that your friend invest in a grammar guide or textbook, which can solve your disagreement and perhaps answer any additional questions. – Theodore Broda May 27 '14 at 14:57
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    But probably won't. Most grammar guides or textbooks give inaccurate rules. For instance, the idea that there can be a direct object in a sentence without a transitive predicate, which your friend seems to have picked up from some grammar guide or textbook. – John Lawler May 27 '14 at 16:16
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The sentence

  • His dog is his best friend.

turns out to mean the same thing when the two NPs -- his dog and his best friend -- are reversed:

  • His best friend is his dog.

These are sometimes called "equational" sentences, because they're commutative, like an equation.
This leaves the impression that is means 'equals', which appears true, but only in this construction.

In fact, both Noun Phrases refer to the same individual, described in two different ways,
and both NPs can be predicate nouns. That's the key to equational sentences.

In the first sentence, his dog is the subject NP and his best friend is the predicate NP.
In the second, where the nouns are reversed, the roles are reversed as well --
his best friend is the subject NP, while his dog is the predicate NP.
In both cases, is does not mean "equals" -- it doesn't mean anything at all.
It's just the auxiliary be that's required with any predicate noun or adjective:

  • His dog is tired. His dog is a dachshund. His dog is his best friend.

Calling any of these sentences a "copula sentence" is wrong, because that term presupposes that the verb be is being used with a special ("copular") meaning here, while in fact it's just an ordinary auxiliary verb, governed entirely by automatic syntax, and totally meaningless otherwise.

  • The final paragraph is a matter of terminology, surely. In the tradition I ‘grew up’ on, copula means exactly what is is here: a semantically empty verb that is required in predicative sentences. Calling it an auxiliary in this usage would be considered wrong in that tradition, because an auxiliary must auxiliate something (a verb). – Janus Bahs Jacquet May 27 '14 at 16:47
  • I grew up on that, too. But be is the verb with a non-verbal predicate. It auxiliates the predicate, but that's not necessarily a verb -- it's quite possible to have a sentence with no main verb, if the predicate is non-verbal. See the Logic Guide and the Verb Phrase Guide. – John Lawler May 27 '14 at 17:10
  • But then what, other than terminology, is the difference between saying that an auxiliary always auxiliates the main verb, while a copula is the main verb and ‘auxiliates’ (for lack of a better word—‘introduces’, perhaps) non-verbal predicates? The end result is, as far as I can tell, exactly the same, terminology aside. – Janus Bahs Jacquet May 27 '14 at 17:15
  • But terminology is also used for description, and the simpler system gives you the simpler description. If you say that is is a main verb in these sentences, then you need to say that some of the properties of main verbs don't apply to it, because it alternates like an auxiliary verb. If you say, however, that it's just an auxiliary verb, you don't get a main verb that needs exceptions stated. And nothing, as far as I know, depends on having a "main verb"; it's a useless concept with far too many instances and contradictory descriptions. – John Lawler May 27 '14 at 17:22
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His dog is his best friend.

You are right--for your example, the subject will usually be considered to be the noun phrase "His dog".

Often, a helpful diagnostic tool is that of creating an interrogative clause for that sentence:

  • [Is] [his dog] his best friend?

Notice that subject-auxiliary inversion has occurred: the auxiliary verb "is" has switched positions with the noun phrase "his dog"; and so, the noun phrase "his dog" can be considered to be the subject.

In general, if the first phrase can function as the subject of the clause, then it's considered to be the subject.

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