Plural or singular noun adjunct?

Question

Wikipedia says:

Noun adjuncts were traditionally mostly singular (e.g. "trouser press") except when there were lexical restrictions (e.g. "arms race"), but there is a recent trend towards more use of plural ones. Many of these can also be or were originally interpreted and spelled as plural possessives (e.g. "chemicals' agency", "writers' conference", "Rangers' hockey game"),[2] but they are now often written without the apostrophe, although decisions on when to do so require editorial judgment.

There are morphologic restrictions on the classes of adjunct that can be plural and nonpossessive; irregular plurals are solecistic as nonpossessive adjuncts (for example, "men clothing" or "women magazine" sound improper to fluent speakers).

Regarding this I'm still not sure which set is correct:

• student welfare officer or students welfare officer
• student union or students union
• student loan or students loan
• sport center or sports center

Can you please provide some guidelines?

Answer 1

The problem here is that the possessive gets confused with the plural. Let's take your second example: "student union" is a noun adjunct and is the most commonly used variant (in the UK at least). Student is singular, as is usual with noun adjuncts. As Wikipedia notes, plurals are becoming more common but personally I find them grating. Most native English speakers instinctively use the singular. On the other hand, "students' union" (note the apostrophe) is quite correct, but it's a possessive of a plural noun, not a plural noun adjunct. An easy way to see this distinction is to look for words whose plural is not formed by adding an s. For example, female police officers and doctors used to be called "woman police constables" and "lady doctors" (note the singulars); but for possessives we have "women's magazines" and "ladies' toilets" using plurals.

Returning to your examples, the first is similar to the second: "student welfare officer" (singular) or "students' welfare officer" (plural, but possessive, not adjunct). Here though, the former, though more natural-sounding, is potentially confusing because it might imply that the officer is a student, rather than being for students.

The same confusion can't arise for your third example. "Student loans" sounds far more natural than "students' loans" but there is still a nuance to think about. The former described almost the institution of given loans to students, whereas the latter might describe some loans given to some students.

Your fourth example raises a completely different issue. The possessive would never be used here, since sport/s is/are inanimate and the centre doesn't belong to it/them. The issue is whether you consider sport in general to be singular or plural, regardless of whether it's used in a noun adjunct. American newspapers have sections called "sports" whereas British ones have sections called "sport."

In summary:

• When using a possessive, use the plural if there is more than one possessor.
• When using a noun adjunct, use the singular.
• But don't use the singular if you would never use it outside a noun adjunct, e.g. an "arms dealer" almost never sells an "arm."
• There is an (alas, growing) list of idiomatic examples where the plural is common, e.g. "people smugglers/"

Answer 2

Unfortunately, I don't think we can. Most of these are based on convention and seem to vary case by case.

The three examples you ask about are:

• Student Union (usually capitalized as the name of a particular building on a college campus)

• student loan

• sports center (uncapitalized, unless used as the name of a specific building, e.g. the Main Street Sports Center, or a television program, e.g. ESPN Sports Center)

But why is it "student union" and not the possessive "students union", while it is "boys club" and not "boy club"? Again, no real reason, just convention, and yet more vocabulary you can only learn by exposure and memorization.