It's simple you use 's in "it's" ="it is" to mark the missing 'i' and you use the 's in eg. "the writer's article" to mark a missing 'e' from "writeres" the ori

It's simple you use 's in "it's" ="it is" to mark the missing 'i' and you use the 's in eg. "the writer's article" to mark a missing 'e' from "writeres" the original possessive which disappeared sometime in late Saxon (6th century) Although "its" is also possessive you don't use an apostrophe because that would confuse its with "it's" Then if the word ends in an s the rules are different unless it's a name, then the rules are different again, unless it's a biblical name then the rules are different to the last rule, except for Moses and Jesus which are exceptions to this. Then of course there are always a few special cases - St James' park (footbal ground) but St James's park (park) Of course if the word originally comes from another language then you have to use the rules from the language instead, well sometimes, say for French but not German or Dutch, actually only sometimes for Dutch depending.