Quote:
This is a reliable guideline, at least for U.S. usage: "'A' plus 'number'" takes a plural verb; "the' plus 'number'" takes a singular verb. So "A number of sources have stated . . .

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Note added at 15 mins (2004-04-20 12:15:15 GMT)
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Еще нашла хорошее объяснение:
http://www.askoxford.com/asktheexperts/faq/aboutgrammar/numb...


Should I write \'a number of people is\' or \'a number of people are\'? Printer Friendly Version


Although the expression \'a number\' is strictly singular, the phrase \'a number of\' is used with pluiral nouns (as what grammarians call a determiner). The verb should therefore be plural: \'A number of people ar waiting for the bus\'.

This is not the case with \'the number\', which is still singular: \'The number of people here has increased since this morning.\'