Because English is a language with very heterogeneous origins, many English words derived from French, Greek or Latin have irregular plurals or even two different plurals, depending on usage. Some words which may appear plural are in fact singular, as in the various names of areas of knowledge. Furthermore, the use of collective nouns and partitive expressions often affects whether the related verb is singular or plural. Finally, despite some notable exceptions, nouns used as adjectives normally revert to their singular form.
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