Could you pleas advise the meaning differences among the words "case study ", "cases study" ’and "case studies "? It’s very helpful if you teach me the differences of the meaning with sample sentenses. Thank you !

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"cases study" is never used.
When using a word that is originally a noun as an adjective, it is almost always put in singular form.

・e.g. "dog shelter" (It's a shelter that contains a lot of dogs, but it is still "dog" as an adjective.)
・e.g. "vegetable drawer" (It's a drawer that contains a lot of different vegetables, but we still say "vegetable" as an adjective.)
・e.g. "pencil case" (It's a case that fits or contains several pencils, but we still say "pencil" as an adjective.)

◎ one study (regardless of whether it's of one case or more than one) → "case study"
◎ multiple studies → "case studies"
...If you wanted to be clear that it was one study on several cases, you could say "a multi-case study"

(余談ですが、数字がついても、形容詞になった名詞は単数形です。
・三連休　→　a three-day weekend
・2時間の映画　→　a two-hour movie
・12歳の男子　→　a twelve-year-old boy　＊twelve years old ではなく)