Recebi um e-mail de um aluno perguntando por que o verbo está no plural na seguinte frase:

“As is the case currently, a large portion of these are in Africa”

Essa é uma pergunta a respeito de subject-verb agreement. O Oxford Guide to English Grammar define subject-verb agreement como “choosing the correct singular or plural verb after the subject“.

Geralmente, a regra é: sujeito no singular, verbo no singular; sujeito no plural, verbo no plural. Entretanto, há vários casos específicos, como o caso da questão feita.

Quando o sujeito é composto por palavras que indicam porções (percent, fraction, part, majority etc.), é o substantivo – ou o pronome – da of phrase que segue que determina se o verbo deve estar no plural ou no singular. Se o substantivo estiver no singular, o verbo ficará no singular; se o substantivo estiver no plural, o verbo ficará no plural. Veja alguns exemplos, retirados do Grammarbook.com:

Fifty percent of the pie has disappeared.

Fifty percent of the pies have disappeared.

One-third of the city is unemployed.

One-third of the people are unemployed.

Isso já explica por que o verbo em “a large portion of these are in Africa” está no plural: ele está concordando com these, que também está no plural.

Entretanto, é importante notar algumas particularidades com relação a essa regra específica, principalmente no que diz respeito a quantifiers. Nesse sentido, o Oxford Guide to English Grammar traz algumas regras, as quais eu organizo na tabela que segue:

Expression	Agreement	Examples
One of	Singular	One of these letters is for you.
A number of The majority of A lot of	Plural	A large number of letters were received. The majority of people have complained. A lot of people have complained.
The number of	Singular	The number of letters we receive is increasing.
Amount of	Singular	A large amount of money was collected.
Amounts of	Plural	Large amounts of money were collected.
(Fractions)	According to the Noun	Three quarters of a potato is water. Almost half the plants were killed.
Every and each and compounds with every, some, any and no	Singular	Every pupil has to take a test. Each day was the same as the one before. Everyone has to take a test. Someone was waiting at the door. Nothing ever happens in this place.
Each followed by a plural subject	Plural	The pupils each have to take a test.
All and some with a plural noun	Plural	All the pupils have to take a test. Some people were waiting at the door.
None of/neither of/either of/any of+ plural noun phrase	Singular or Plural (Singular is more formal)	None (of the pupils) has/have failed the test. I don’t know if either (of these batteries) is/are any good.
No	Singular or Plural	No pupil has failed/No pupils have failed the test.

Cheers!

Sources:

Grammarbook.com

Eastwood, John. Oxford Guide To English Grammar. Oxford: Oxford University Press, 2002.