noun
1
BAD : The police was not able to find anything.
GOOD : The police were not able to find anything.
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Police is always used with a plural verb: 'The police have a very difficult job to do.'
2
BAD : He was charged with shooting a police.
GOOD : He was charged with shooting a police officer.
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the police = the police force in general: 'If you get any more of these phone calls, you should contact the police.'
policeman, policewoman, police officer = a member of the police force: 'There are two police officers outside waiting to see you.'