From “Beating a retreat” in The Economist, 9/24/11, p. 99 (on-line here):
… soot particles absorb sunlight, and so warm up the atmosphere. Then, when snow or rain wash them onto an ice floe, they darken its surface and thus cause it to melt faster.
This is 3sg or 3sg (snow or rain) functioning as 3pl for the purposes of subject-verb agreement (wash rather than washes), though a general principle –
(1) When all parts of a subject joined by or or nor are singular, the verb is singular; when all parts are plural, the verb is plural (Little, Brown Handbook, quoted in “Agreement with disjunctive subjects”, here)
would predict 3sg agreement (and I would have used 3sg in this case).
Intuitively, this is a kind of “notional agreement”, snow or rain being understood as ‘snow and rain, whichever happen(s) to occur’. This is an unusually simple example; in the other sg or sg = pl cases I’ve collected, other things are going on.
Background. The overarching principle here (from “Agreement with disjunctive subjects”) is:
Disjunctive Agreement (DA): with a disjunctive subject, the verb agrees with each of the disjuncts.
DA covers (1) and also predicts a conflict when the disjuncts are of different persons and/or numbers (as, for example, in the subject either you or I). How do people handle these conflicts?
There is quite a lot of variation. For some speakers, agreement is blocked; no verb form is satisfactory. Most speakers allow a resolution of the conflict: by Agreement with the Nearest Disjunct (AND, giving either you or I am); by Agreement with the Higher Person (AHP, giving either you or I are); by Agreement with the Higher Number (AHN, giving, for instance, either we or she are); or by defaulting to the unmarked (3sg) form (giving either you or I is). Many speakers allow more than one resolution, as alternatives. Details in my LLog piece.
But when the disjuncts are of the same person and number, there’s no conflict for DA, and we should get (1). But that doesn’t always happen.
One set of cases came up in my postings “More variation than expected” (here) and “He or she are” (here): subjects of the form (either) he or she can work, for some people, just like “singular they” (singular in semantics, but taking plural agreement). From the latter posting:
It seems pretty clear that some people can use he or she, at least some of the time, as an alternative to singular they, and with the plural agreement appropriate for they. No doubt a testimony to the durability and naturalness of singular they.
Beyond this case, I’ve slowly been collecting some other examples. Here are two:
“A Republican who only talks about border control or a Democrat who only talks about benefits and services for illegal immigrants are going to find themselves in trouble next fall,” said Dan Schnur, a Republican stragegist who worked on Senator John McCain’s presidential bid in 2000. (Michael Luo, “Walking A Tightrope on Immigration”, NYT Week in Review 11/18/07, p. 1) (link)
But getting [Paula] Deen to unplug the waffle iron doesn’t strike to the core of the problem any more than posting fast-food calorie counts or taxing soft drinks do. (Frank Bruni, “Unsavory Culinary Elitism”, NYT 8/25/11, p. A23) (link)
These two could be seen as examples of “agreement with the nearest” (AWN), in particular the subtype of AWN in which postnominal expressions that end in a plural noun trigger plural agreement on the verb, even though the modified noun is singular:
… or a Democrat who only talks about benefits and services for illegal immigrants [pl verb are]
… or taxing soft drinks [pl verb do]
On the other hand, they could also be seen as examples of or “understood conjunctively” (like snow or rain in my original example):
a Republican who … or a Democrat who … ‘a Republican who … and a Democrat who …’
posting fast-food calorie counts or taxing soft drinks ‘posting fast-food calories and taxing soft drinks’
As before, I would have used a sg verb, but there seems to be some inclination for people to opt for notional agreement rather than formal agreement.