Ad Sensum Agreement and Distributive Readings
Spanish, as well as other Romances languages (like Rumanian), exhibits an apparently optional agreement in number between subject and verb when the subject is headed by a collective noun:
(1) Un grupo de niños cantó en el concierto
(2) Un grupo de niños cantaron en el concierto
This situation is restricted to this type of nouns, and only happens if the collective noun is in singular. If the noun is in plural, the agreement is mandatory:
(3) Dos grupos de niños cantaron en el concierto
It is interesting that, as has been observed, when the singular collective noun triggers plural in the verb, a distributive reading is forced:
(4) Un equipo de nadadores recibieron una medalla
(5) Un equipo de nadadores recibió una medalla
So, (4) means that each of the members of a swimming team received a medal, whereas (5) means that there was only one medal for the whole team. Notice further that if the prepositional phrase (with a plural noun) is not present, the disagreement is not possible:
(6) * Un equipo ganaron una medalla
This means that this prepositional phrase (and the plural noun) is the trigger for the distributive reading. It is tempting to conclude that it is responsible for the agreement too. However, and this is an even more interesting data (although less known), some time others phrases can trigger the distributive reading too:
(7) Pasan uno a uno el convoy de la guerra
In (7), the phrase uno a uno (‘one by one’) triggers a distributive reading over el convoy de la guerra (‘war convoy’), as expected, the verb is in plural; however, convoy is singular, and there is no prepositional phrase with plural noun.