For < s >, it is used to indicate a plural noun. e.g. Houses, tables, cars, buildings For < 's >, it is used to indicate possession of a person/thing. e.g. Sally's books, the table's legs < 's > is also used as the 'short form' of 'us' e.g. Let's set off (which means 'Let us set off') < 's > is used as the 'short form' of 'is' as well e.g. That's it (which means 'That is it') For < s' >, it is used to indicate the possession of a plural noun or a name that ends with 's'. e.g. the tables' legs, Marcus' books I hope this is helpful for you. (:

When a noun ends with the letter "s", for example, parents, teachers, abacus, etc...its possessive case should be ~~~~s' Otherwise, the noun's possessive case should be ~~~~~~'s o(∩_∩)o