In this sentence:
The waiter refused service
the subject `the waiter’ did not perpetrate any act upon `service’. So the noun `service’ is the complement in this sentence. It instructs the copular verb `refused’ in the content of the subject's act, which is `service refusal'.
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Where there is a verb there can also be an adverb. There is no verb in copula and copular-verb sentences. There cannot, therefore, be an adverb in copular and copular-verb sentences.
(From: http://www.englishgrammartutor.com/The%20Parts%20Of%20Speech.htm#copular%20verb
I took a look at that site and found it decidedly quirky. A fair number of statements there show that the author has quite an idiosyncratic way of approaching grammar, not to mention that he has an axe to grind (in the final paragraphs). There may be a few interesting and creative ideas in it, but they are too undeveloped in their present state to rely on for any really good practical advice.
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