+ Get exercises on 's:
 the film's ending / the armies' leaders
 Creepy Doll / How to Eat Slugs
The Rules in Brief
1. -'s shows that something belongs, e.g. the kid's closet;
2. you can use 'of' instead, e.g. the closet of the kid;
3. if the word already ends in -s, use -s', e.g. the kids' closet;
4. 'its face looks weird' BUT 'it's weird' = 'it is weird'
The Rules in Detail
Normally, I teach -'s by starting with -s. The fact they sound the same is what causes some of the confusion. Also, both are 'word endings' that get added on to show an extra bit of information.
In English, -s is used to show plurals (pl.) (more than one).* e.g. kid > kids (pl.)
*not all plurals end in -s, e.g. men, mice, fish.
1. -'s is to show that something belongs. We call this 'the possessive'
e.g. the kid's closet, the kid's shoes, the kid's hopes and dreams
You can rewrite these as:
'the closet of the kid', 'the shoes of the kid', 'the hopes and dreams of the kid'
I think the -'s version works best in the examples shown above.
2. -'s and 'of' are both ways to write the possessive.
e.g. London's main attraction is its history.
OR: The main attraction of London is its history.
e.g. The society's main purpose is to promote animal welfare.
The main purpose of the society is to promote animal welfare.
This also works the other way round:
e.g. It took many hours of hard work to build the bridge.
OR: It took many hours' hard work to build the bridge.*
e.g. The plan was the result of many days' hard work.*
*find out why these are -s', and not -'s, below.
3. How do I use an apostrophe if the word already ends in (s)? e.g. most plurals, or words like 'James', 'bus', or 'crisis'. Put the apostrophe after the (s) > -s':
e.g. James' ability to squawk like a duck was the result of many hours of practice. (or: many hours' practice)
e.g. The swans' wings were streaked with oil.
Bear in mind (s)+(s) is nearly impossible to say. So normally people avoid it with tricks like this.
e.g. The bus' seats were filthy. > The seats on the bus were filthy.
e.g. The crisis' worst moment was... > The worst moment of the crisis was...
4. FINALLY, but very importantly: its/it's
Possessive pronouns - its, his, her, their, our, my - never take an apostrophe.
e.g. I don’t know why its lid came off.
Its face is really creepy.
'It's really creepy,' has a totally different meaning to the above. I'll post on rules for these apostrophes soon.