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Sometimes words that are usually used as nouns are used as adjectives.

Ex : The shoe store also sells books

When a noun is used as an adjective, it is singular in form, NOT plural. But I found some english documents that contains some identical structure like : sales assistant, clients need. In this case, why are "sales" and "clients" plural in form ?
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Usually it's singular. "Sales" is a mass noun - "I'm in sales" means that you sell for a living.

With "clients' needs" or "clients' needs" it should be possessive.
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AnonymousWhen a noun is used as an adjective, it is singular in form, NOT plural. But I found some ...
Yes. You'll find a few plurals there, but 99.9% are singular.
CJ