How do you use plural or singular form of an adjective?

Question

So I was taught that you should almost always use singular form of an adjective instead of plural form. But recently, I've seen people using plural form and it sounds quite weird and I don't know if it is correct.

For example,

• "Salary tax" instead of "Salaries tax"
• "Office supplies" instead of "Offices supplies"
• "Product demo" instead of "Product demo"

In the above cases, the word "Salary" is used as an adjective to describe the type of "tax", so it will be in singular form. But I saw people using salaries tax and not sure if it is grammatical.

So my question is if there are any general rules on how to use the singular/plural form of an adjective?

Answer 1

As snailplane and Man_From_India tell you, your question does not involve adjectives, which never have a distinct plural form, but attributive nouns.

The singular form is certainly the ‘default’ for attributive constructions, but plural attributives are not uncommon. Some of these (and probably most of the older ones) come about because the singular and plural forms have different meanings. For instance

• We take an ‘arts degree’ or a ‘humanities degree’ because the singular would imply something different: an ‘art degree’ is a degree in painting or sculpture or something of the sort rather than a degree from the School or Faculty of Arts, and a ‘humanity degree’, would be a degree from the Department of Humanity, if such a thing existed.
• We speak of the ‘civil rights movement’ because ‘civil rights’, plural, is a term with a specific meaning, the entire body of rights supposed to be held by all members of a civil society rather than a specific civil right or a general desire for ‘rightness’ in civil society.

In other cases, the plural seems to have come about as a sort of compromise where the underlying sense might represent a simple plural or a singular possessive or a plural possessive—see, for instance, this blogpost on the correct spelling of Veterans Day. (But this is by no means a rule; see this Google Ngram on various names for the laws under which US workers are compensated for on-the-job injuries.)

In yet other cases, grammatical concord seems to be in play. We speak of Virginia Woolf as a woman writer, but we are more likely to call Woolf and Eudora Welty and Flannery O’Connor women writers than woman writers. Geoffrey Leech, in Change in Contemporary English, Cambridge, 2009 (220) suggests that this is more likely when the plural is irregular, without an -s.

But none of this explains why plural attributives became much more common in the second half of the 20th century, particularly in British English. Leech suggests that:

The increase of plural attributive nouns may well have been brought on by the general increase in noun sequences, bearing in mind that the implicit link between two adjacent nouns has to be inferred, putting a burden on the cognitive processing abilities of the reader. In such circumstances an -s at the end of a word may be a handy clue to interpretation. Thus the generic meaning of the plural in weapons purchases or fisheries protection is helpful in eliminating the possibility of a singular interpretation (that one weapon is to be purchased, or that one fishery is to be protected) and in capturing the intended generic meaning of the whole (220-221).

Leech says that Stig Johannson, Plural attributive nouns in present day English, Univ. Lund, 1980, identifies more factors, but this does not appear to be available free online.

Answer 2

Late to the ballgame here. I'm an editor. StoneyB's answer is excellent, but the distinction between adjective and "attributive noun" seems a little fussy in this context--especially in helping someone who identifies as an English learner. I had gone my whole life until now without ever hearing the term "attributive noun." The words in question are being used as adjectives, and "offices supplies" "knives set" "cacti garden" are wrong and weird-sounding because the noun is functioning as an adjective, and therefore typically shouldn't be pluralized--at least in American usage; dunno about the Brits.

The far less common plural ones (arts degree, data set) exist for the reasons StoneyB points out, but also because people just get tripped up by irregular plural forms and don't know what to do--they guess wrong, erring in the direction that they think sounds more sophisticated (especially if in a corporate or academic setting), and then the new irregular construction gets accepted into the language (as alluded to by the Geoffrey Leech in StoneyB's answer). For example, I ended up here because I was searching the internet for a good explanation of why I've come across the incorrect "consortia assessment" multiple times in my work. Nobody would say "consortiums assessment"--it's too easy to see that it is wrong. And so it is unlikely to enter into the language. I hope.

Answer 3

Have to agree with StonyMan, but living with a lot of English learners I hear a lot about "Cars Park" and "Guns control", constructions I actually like though they sounds weird. My friend was adamant that "Car Park" makes as much sense as "Man Room" In thinking about this, what came to mind was that this phrase doesn't use a "plural" but a plural possessive i.e. "Men's (and Ladies') Room". My friend wasn't troubled by this since to him "Men's" sounds like a proper plural.

Answer 4

Salary tax, product demo, office supplies are all examples of compound nouns. Here is a link for further study.

In your examples compound nouns are created using "Noun + noun".

So in Salary tax, product demo or office supplies, there is no presence of adjectives. All are noun.

And an adjective can never be singular or plural. I mean there is nothing like that.

Please let me know if my answer addressed your question.