pls explain why sometimes I have to use 's and other times it is s'
You always put the apostrophe at the end of the word and the s after that, but if there is already an s at the end of the word you can leave it off. So if the word ends in s (including plurals) just an ', if it doesn't then add 's. For Example: The boy's ball was lost. (One boy owned a ball which was lost) The boys' ball was lost. (More than one boy owned a ball which was lost)
"The Complete Plain Words" by Ernest Gowers is always worth having on the bookshelf for situation like this ...
The apostrophe S shows possession (Charles’s ball), but if the word is a plural that ends in S we put the apostrophe after it (parents’ house) - hope that helps!