I'm reading a book on America accent and there's a page with exercises.


Circle the function words in the following sentences:

  1. The sky is blue.
  2. ...
  3. ...

The answers are provided at the end of the book. For the first sentence the answer is the word "is".

If I understand correctly, helping verbs and auxiliary verbs are function words, but in the sentence above the word "is" is a normal verb.

I think the only function word in my sentence is the article "the". Am I right?


In traditional grammar the verb BE was considered as a main verb (or lexical verb) when used on its own in a sentence. It was only considered an auxiliary when it was used as part of a passive construction or a continuous construction. However, we now understand that auxiliaries are a grammatical class of words that have the same grammatical properties. Lexical or main verbs do not share these properties. But the verb BE usually does, even when it is the only verb in the sentence.

Auxiliary verbs

The important central properties which characterise auxiliary verbs are sometimes referred to as NICE properties. NICE is an acronym for:

  • Negation
  • Inversion
  • Code
  • Emphasis


Auxiliary verbs are necessary in English to make negatives with not. They also have a tendency to contract with the word not. If there is no auxiliary in the positive version of the sentence we use the dummy auxiliary DO:

  • The giraffes can't stand up.
  • We don't know what to do.
  • They mustn't leave.
  • The elephants aren't happy.

In the sentences above we see the negative particle occurring attached to the auxiliaries. When not contracted it must occur directly after the auxiliary in canonical negative sentences. Notice how the verb BE is fulfilling this function even though there is no following verb.


There are several instances in English when we need to invert the subject and auxiliary verb. The most obvious is in yes/no questions. Obviously we can only do this if there is an auxiliary! When there is no normal auxiliary in the declarative counterpart sentence we use the auxiliary DO:

  • Can the giraffes stand up?
  • Do we know what to do?
  • Must they leave?
  • Are the elephants happy?

Notice again that the verb BE is behaving just like the other auxiliary verbs.


Sometimes we delete material following an auxiliary verb leaving the auxiliary to stand in for the missing material. In answer to the questions above we can give the following answers. The missing material is in brackets:

  • They can! [stand up]
  • We do. [know what to do]
  • They must! [leave]
  • They are. [happy]

Again BE is not an exception here.


When wanting to add positive emphasis to a sentence, underlining that the sentence is true, we add emphasis by stressing the auxiliary, thereby giving it a full vowel (and preventing any contractions with subject pronouns). If there is no auxiliary in the canonical sentence we need to use the auxiliary DO:

  • The giraffes can stand up.
  • We do know what to do.
  • They must leave.
  • They are happy.

We can see then that BE is a member of this auxiliary verb family.

The Original Poster's question

The Original Poster is right that the word the is a function word.

Because it is an auxiliary, the verb BE should be regarded as a function word too. The Original Poster is a skilled English pronunciation enthusiast (this question comes from a book on pronunciation). The verb BE behaves like a function word in terms of pronunciation too.

The verb BE, like many other function words usually doesn't take stress, although like other auxiliaries it is always stressed when part of a negative contraction:

  • He is 'happy
  • He isn't happy

Like other auxiliaries it is often stressed when stranded at the end of a clause - even when it is not negative:

  • Yes, they were.

Like other function words, the different grammatical forms of BE usually have two pronunciations. There is a normal one (referred to by linguists as the weak form) for when the word is not stressed and not stranded. There is also a strong form for when these words are either stressed or stranded or both. For example the word am, when not contracted is usually pronounced /əm/. But if it is the last word or if it is stressed, it is pronounced /æm/:

  • Whose coming? I am! (/æm/ because it is the last word, even though it is not stressed here)
  • I am coming! (/æm/ because stressed in emphatic sentence)
  • I alone am able to help you. (/əm/ in normal unstressed occurrence)

It is useful then in terms of English phonetics and phonology to regard BE as a function word.

  • 1
    Thank you for the detailed answer. Your time is greatly appreciated. Jun 9 '15 at 14:25
  • @ZoltanKing You're welcome. You might want to unselect my answer though, at least for a day or two - you might get a much better one! (and more people will read this question thread!) Jun 9 '15 at 14:37

The short answer is no, you're not right. If you count auxiliaries as function words, "is" must be a function word, because it's an auxiliary. See Araucaria's answer for why "is" is an auxiliary. I thought I would just add a note here about how grammarians reason about such things.

People trained in traditional grammar tend to make nominal arguments about such matters, i.e., arguments based on what a thing is customarily named. So, it may seem that "is" in your example, "The sky is blue", cannot be an auxiliary verb, simply because there is no other verb around that it could be auxiliary to. Is that a sensible argument? Not really. Because it assumes that the term "auxiliary" is a perfectly appropriate term, and that may not be so.

The tendency in modern grammar is to use grammatical terms in a grammatical sense, rather than in a semantic sense. We count "is" in your example as an auxiliary because it displays certain grammatical properties in common with other words called "auxiliaries", as Araucaria goes into some detail about. The fact that this "auxiliary" can occur where there is no verb to which it could be auxiliary means that this old term, "auxiliary", is no longer perfectly appropriate. But this is a terminological problem, not a grammatical problem.

You can't show that "is" is not an auxiliary by showing there is no other verb around. All you can show that way is that to refer to it as "auxiliary" is misleading. But how you refer to it doesn't affect what it actually is. We should resist the temptation to make nominal arguments.

  • 1
    Verbs, adjectives, and nouns can be predicates in English, but only verbs can carry tense markers. Be is the auxiliary verb required before predicate adjectives and predicate nouns. The adjective or the noun is already the predicate, but can't be the verb, and there hasta be a verb to carry the tense, so there's an auxiliary be for that purpose. Very simple. There is also an equative construction A is B, which is usually just for identifying two NPs as coreferent, and this is the "content" usually given to be in traditional grammar. But it's just another construction.
    – jlawler
    Jun 9 '15 at 16:43
  • 1
    @jlawler, no, nouns can't be predicates, only NPs. And you forgot PP. If the only function of "be" before a NP is to carry the tense, why can't we just have nothing there when tense is the present, as in Russian? Or why can't we say "Bush ex-president", since the past sense is expressed within the noun? (But I don't want to seem argumentative.)
    – Greg Lee
    Jun 9 '15 at 17:06
  • OK, NPs, APs, and PPs. The point is that be is an auxiliary in those cases, not a lexical verb. Why we need it is -- agreed -- a dicey matter.
    – jlawler
    Jun 9 '15 at 17:16
  • 1
    I agree that auxiliaries are a different part of speech from verbs. That was the analysis of Chomsky's Syntactic Structures, was argued against by Ross (Auxiliaries as Main Verbs), was defended by Jackendoff. In my version of relational grammar, the various auxiliaries of English bear different grammatical relations to the (real) verb.
    – Greg Lee
    Jun 9 '15 at 18:26
  • I wouldn't call auxiary verbs a different part of speech, just a variety of verbs that have been bleached mostly clean of semantics and hung on the grammatical apparatus. If there's a category Auxiliary, it ought to also include articles, aux verbs, governed prepositions, phrasal verb particles, and probly a lot of other things. They're all essentially crutches for lost morphology. I never got into RG deep enough to deal with grammatical relations of one verb to another, so I can't talk about that; for me, grammatical relations are between predicates and argument NPs.
    – jlawler
    Jun 9 '15 at 19:00

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