Sometimes the "'s" method is not really possible. This is when the possessive has the "of the" construction.
For example: "The Queen of England" is better than "England's queen."; "A day of the week" is much better than a "week's day".
With plurals, there is already an "s" on the end of the word. But we do not want to write possessives with "s's" on the end. It is a waste of one "s" and it makes pronounciation more difficult. So we just put the apostrophe after the "s" instead of before.
For example: The nurses' home (more than one nurse lives there)/ The nurse's home (one nurse lives there.) : The computer's keyboard (one computer, one keyboard)/The computers' keyboards (more than one computer, more than one keyboard).
So, do you think you understand nouns? Ok then - here are some exercises for you! Click the blue triangle.