noun
1
BAD : The dancers were dressed in their national clothes.
GOOD : The dancers were dressed in their national costume.
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c ostume = (1) (countable) a set of clothes worn during a performance by an actor, clown, etc: 'She used to work for a theatre company, designing and making costumes.' (2) (uncountable) a set of clothes that are typical of a particular country or historical period: 'The castle guides were dressed up in Elizabethan costume.' 'A group of Hungarian folk dancers came on stage, all wearing national costume.'
2
BAD : Don't go out and buy a special clothes.
GOOD : Don't go out and buy any special clothes.
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Clothes is a plural noun: 'I need some new clothes.' (NOT 'a new clothes')