boys is the ordinary plural: One boy Two boys boy's is the possessive singular: The boy's bedroom = The bedroom belonging to the boy boy's can also be short for "boy is" or "boy has": The boy's a genius = The boy is a genius The boy's run away = The boy has run away boys' is the possessive plural: The boys' bedrooms = The bedrooms belonging to the boys (plural) It's similar for other nouns, but there are some special rules, for example if the noun itself ends with an "s" (or certain other characters). There is a detailed explanation at I don't understand what this has to do with hyphens (your subject line). Perhaps you got mixed up: the character ' in boy's and boys' is called an apostrophe.
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