Function words

[zoltankr]

Hello, I'm reading a book on American Accent and there is a page with exercices.

Circle the function words in the following sentences:
1. The sky is blue.
..
..

On the page with answers the author underlined "is" in the first sentence. However for me, the word "is" looks more like a normal verb. It's not a function word (auxiliary or helping). I think the only function word in the sentence above is the article "The" Am I right?

[panini]

Function words are traditionally opposed to content words, which have contrastive lexically-idiosyncratic meaning. The copula on the other hand only specified that the post-verbal expression is predicated of the subject, that is, it only expresses a structural function. But "is" is a "normal verb" in the sense that it doesn't require a following verb (as auxiliaries do). Since "function word" is not a technical concept, there's really no way to decide what the copula is until you specific a particular metatheory.

[Copernicus]

Copular verbs are very interesting from a syntactic perspective, because they behave just like auxiliary verbs in languages that have auxiliary verbs.  For example, they participate in subject-auxiliary inversion patterns in English questions, and they can precede "not", unlike regular main verbs.  This is not just true of English, but of all languages that I am aware of.  Although the predicate phrase following the copula is clearly not a main verb, it, too, can behave syntactically like a main verb in some languages, e.g. Breton.  So I would definitely treat it as a function word in English.

ETA:  This topic probably belongs in the morphosyntax forum rather than phonetics and phonology.