I previously thought making a noun into possessive form was as simple as adding an apostrophe-s ('s) if it does not end with an "s", and just a simple apostrophe if it does. I was tutoring my Grade 4 daughter on possessive nouns last night and I was surprised at all the other rules.
Rule #1 - If the noun is common and singular, always put apostrophe-s -- even if the noun already ends with an "s" already. So for example, you should say "princess's gown" instead of "princess' gown".
Rule #2 - If the noun is proper and single syllable, always put an apostrophe-s also except if the word following it starts with an "s". For example, its "James's car", but "Kris' shoes".
Rule #3 - If the noun is proper and multi-syllable, then you follow the standard rule of apostrophe-s if it does not end with an "s", and just apostrophe if it does.
Wow! I learned something new from 4th grade!