Consider this example:

Commercialese is an instrument of art, designed to enrich and invigorate our language—surely you will all agree with this—, and we should encourage newcomers to learn it. However, a side-effect is the spread of commercialese to other domains. This we must object to, because we need to keep this precious instrument in our exclusive possession.

How do we determine what the subject complement is in the second sentence, "a side-effect" or "the spread"? In this case you might say it doesn't really matter.


Here are a few examples from Fowler's:

The only comment necessary ... is that, when ... it makes no difference to the meaning which of two words is made the subject and which the complement, the one that is placed first must (except in questions) be regarded as subject and have the verb suited to its number: Our only guide was the stars, or The stars were our only guide. Such apparent exceptions as Six months was the time allowed for completion / The few days Mrs. Kennedy will spend in London is in the nature of a rest for her, are not true ones, for here the complement makes it clear that the subject, though plural in form, is singular in sense (*a period of — *).

So we are to take "six months" as singular because we can imagine it to be an attribute to an omitted, singular head noun, "a period of". But couldn't we imagine this for many other sentences? Consider "the stars were our only guide": I could say "the light of the stars was our only guide". So how is "six months" much different? Isn't there inversion at work here? And is there no way to pinpoint a subject, apart from its position before the verb?

  • I've added to my answer a little following your edit. Just to clarify (before I spend time answering the wrong question) - when you ask in your very last sentence how to pinpoint the subject, what sort of situation are you talking about? Is it analysing the grammar of an existing sentence, or putting a concept into words, or somewhere in between (e.g. deciding how to make the verb agree in a sentence of the form "X is/are Y")? – psmears Jan 22 '11 at 13:18
  • @Psmears: That's a nifty trick, with the "six months seems like..."; I hadn't thought of that. As you say, this does rule out subject-complement inversion. Mere analysis was my goal, because this has always bugged my in all the languages I know. Even Greek and Latin have this analytical problem of determining verb; and, because word order often works in a different way there, we lack the instrument of a fixed order S-V-Sc. Because this analysis is so vague, I was desperately trying to find the odd sentence where it would matter, hence the wrong example. ... – Cerberus_Reinstate_Monica Jan 22 '11 at 13:53
  • ...I know most about analysis in Latin; for that language, I might try to define the subject according to functional roles, as in [Pragmatics][1]; an hypothesis could be that subjects are more likely to contain topical/referential information, whereas s. complements contain focal/new information. Complements are part of the predicate, which is usually considered to be more focal. If true, labelling s. and s. c. would not be meaningless. Then there will be several other factors that may force the hypothesis into bends and exceptions. [1]: en.wikipedia.org/wiki/Pragmatics – Cerberus_Reinstate_Monica Jan 22 '11 at 14:08

First, note that "x is y" is not always logically equivalent to "y is x". For example, "Fools are my friends" is different from "My friends are fools" (because the first allows wise men to be my friends too, whereas the second does not); "All men are mortals" is very different from "Mortals are all men" :-)

That said, sometimes there is an equivalence, and some ideas can be expressed either way. In these cases the verb will agree with whichever you choose to make the subject ("One side-effect is headaches"; "Headaches are one side-effect").

(Note that in your examples, "**Our goal were the mountains" is wrong - in standard writing it should always be "Our goal was (complement)". But you could say "The mountains were our goal", with the same basic meaning but different emphasis.)

Addressing your second question: wholesale inversion of a "to be" sentence (where a sentence of the form "noun copula complement" changes to "complement copula noun") is rare, except for in specific situations:

  • Questions (because English likes the question word to go first: "Who is Fred? Fred is the tall man"). The inversion always happens unless there's a specific reason not to (perhaps expressing surprise - "He's whose brother?")
  • Certain comparative expressions ("Better still are the ones that follow"). The inversion here is optional ("The ones that follow are better still" is fine too).
  • Expressions describing location. The simplest in this category are "There is...", "Here are..." and friends, but I'd also include in this category "Next to my house are two restaurants", "Found in every city are cars and buses". This inversion is optional.
  • Poetic effect - either for emphasis, or for reasons of metre/rhyme ("Blessed are the meek" - the natural phrasing is "The meek are blessed"; the inversion serves to imply a very great level of blessedness; "Old King Cole was a merry old soul, and a merry old soul was he" - the inversion is clearly unnecessary but nicely fills up the line)

Note that in these situations (except possibly the last), it is usually very clear from the construction that the word or phrase at the start of the sentence is not the subject - often because it's an adjective or adverb phrase. In situations where the distinction is not so clear - such as the examples you provided - inversion will rarely if ever be used (since the sentence tends to end up sounding plain wrong, rather than inverted).

Do note that there are a large number of situations where a slightly different form of inversion ("noun verb complement" changing to "verb noun complement") is used - where it is forced or allowed by the use of certain forms or expressions. Examples such as "Is he tall?", "Never am I angry" are examples of this second kind. I won't attempt to enumerate these, because for one thing there are a lot of them, and for another I don't think the uncertainty you're concerned with arises here. In any case, here is a list of uses of inversion, that contains both types.

[Edit to respond to edit in question]

I have to agree with Fowler about the time periods: the "six months" is considered a single unit, so the singular is used; this is common when referring to measured quantities:

  • Ten pounds is a small amount to pay.
  • Two litres is more than enough.

We can tell that this is not inversion by using a verb where the ambiguity doesn't arise:

  • Six months seems like an eternity.
  • Five dollars buys me a very nice lunch.

We can construe the sentence like Fowler as meaning "A period of ...", "An amount of ...", or equivalently by considering the phrase as meaning "Six months of time", "Five pounds of money"; mentioning the uncountable noun makes the reason for the singular clearer, and distinguishes this case from the "light of the stars" case, where there's no obvious way to do the same.

(The plural can sometimes also be used in these cases, giving a sense of referring to each of the individual items mentioned "The six months are dragging on slowly" emphasises that every single one of them is felt.)

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  • +1, for lots of good things. Agreed in particular that *Our goal were the mountains sounds quite wrong (to this native BrE speaker, also familiar with AmE). And I’d tentatively agree that a side-effect is the subject in your other example, since it doesn’t fall under any of the classes of inversion given and also because the in the analogous headaches example, I would definitely expect A side-effect is headaches not *A side-effect are headaches. – PLL Jan 15 '11 at 5:30
  • Yikes, I am sorry my reply comes so late! I needed to look something up in Fowler's, but I didn't have it on hand; then I forgot to reply, even though I had already done so in mind, when I had nothing to do on the train. Thank you for this excellent summary. When I read back my mountains example, it looks plain wrong; I think I was all dizzy from pondering the "essential" qualities of subjects v. complements. – Cerberus_Reinstate_Monica Jan 22 '11 at 2:09
  • ... So your position is to call that constituent subject that comes before the verb. I think Fowler agrees with you. So is there no other quality that distinguishes subjects from subject complements? I think my mention of inversion was based on something I read long ago, but I don't remember. What about these, from Fowler: "Six months was the time allowed for completion / The few days Mrs. Kennedy will spend in London is in the nature of a rest for her."? I will edit these into my question. – Cerberus_Reinstate_Monica Jan 22 '11 at 2:18
  • @psmears: Many thanks for an interesting and informative answer. But that "light of the stars" does my head in when I think about it. It just doesn't make real-world sense to me for that to be a singular 'guide', because you can't be guided by starlight any more than by a hand-torch. Forgetting 'the pole star' for now, you can only be guided by the relative positions of multiple stars which allows you to orient yourself. So surely they must logically be pluralised as 'guides'? – FumbleFingers Jun 5 '11 at 22:11
  • @FumbleFingers: I see what you mean (and I agree about many of the examples addling one's head after a while :), but I have no problem with this phrase meaning "the light of the stars (and its attendant brightness, direction, distribution etc.) is my guide" - though perhaps "the light of the stars is my map" would fit this interpretation better... – psmears Jun 5 '11 at 22:19

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