This question already has an answer here:

If we consider them as plural nouns and sometimes as singular nouns.

marked as duplicate by user140086, RegDwigнt Jul 1 '16 at 9:04

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  • I voted to close your question as "unclear". You need to show us some example sentences, context and your own research. Please make sure you take the tour and visit our help center for additional guidance. Unless you edit your question, this question will be closed. – user140086 Jul 1 '16 at 8:32
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    This is really two entirely unrelated questions mashed together. One of them a duplicate (see the linked question), and the other just common sense: just how many sheep do you mean? One, use singular. Not one, use plural. General reference. – RegDwigнt Jul 1 '16 at 9:05

Police is generally considered to be plural (according to the New Oxford American Dictionary). Therefore, you should say “The police are coming.” rather than “The police is coming.”.

Army is just singular: one army, more armies.

Words such as ‘sheep’, ‘deer’, ‘fish’, etc, just happen to have the same forms for plural and singular. Thus if you speak of one sheep, you use a singular form: “The sheep walks.”, but if you mean more than one sheep, you use a plural form: “The sheep walk.”

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