I think that the possessive cases of nouns and pronouns have some of the most ridiculous rules in English. For example, around third grade we all learned that when you make a noun possessive, you add an apostrophe and the letter s. That’s simple enough; so, you can say the following:
That is Edgar’s porcupine.
The girl’s shoes do not match her outfit.
The dog’s ball is under the chair.
Then it gets a little trickier. Let’s say that the noun is plural. We have to make the noun plural by adding s, es, or ies, and the we put the apostrophe after the letter s rather than in front of it. So now you can write things like:
The monkeys’ bananas are purchased fresh daily.
That is the Smiths’ new RV.
The plumbers’ union may go on strike.
Okay – so far, so good. But then your teacher threw you a curve ball. Even though we all learned that a noun can be replaced with an equal pronoun (meaning it reflects the same number and same gender as the noun), pronouns have their own case of possessives, and they never – I repeat – NEVER have an apostrophe in them. So, now we can write things like:
The dog chased its tail. (That’s the possessive pronoun its. If you use it’s, you’ve written the contraction of it is.)
Those cupcakes are hers.
The house that got broken into was theirs.
I find that about half my students can’t resist the urge to add apostrophes to possessive pronouns. Of course, they also add them to plural nouns on occasion, too. I had a good ten minute discussion one evening with a student who insisted that its’ was the plural possessive of the pronoun it.Hmm. The wonders of public school education.
Now comes the fun part. Probably 75 to 85 percent of my students have never heard the following rule, and I confess that I was in college before anyone explained it to me. Technically speaking, in the formal rules of English, only living things can be possessive. Humans, animals, plants, alien life forms – all of these can be possessive. Inanimate objects can’t be possessive. Therefore:
The tree’s leaves have all turned red – is a correct sentence. However,
The car’s horn was really loud – is incorrect. The proper sentence would be:
The car horn was really loud – OR – The horn of the car was really loud.
Of course, because this is English, we have to have a plethora of exceptions, so here is the first one. Time, and any element of time, can be possessive. Why is this? Who the heck knows? I’ve actually researched this one and I can find no rhyme or reason to it, but this is really the rule. So now we can write things like:
I earned two weeks’ pay.
He called on a moment’s notice.
That is last year’s model.
It’s too early to declare that one of this century’s best movies.
One place where this gets very confusing for people is when we use the numerical identification for a decade. Everyone seems to want to add an apostrophe, whether or not it’s being used in a possessive form. For example:
During the 1960’s, many young people became interested in politics. – X
This is wrong because nothing is being associated with the decade in terms of possession or affiliation.
The 1960’s muscle cars still fetch a handsome price at auction. √
Here the apostrophe is correct because the cars are being associated with that decade.
“But wait,” I can hear you saying. “I see things like (fill in your exception here) all the time.”
Yeah, me too. A casual exception to this formal rule is that anything associated with natural elements or our planet could be considered living to some extent. It’s a good thing, too, because that allows us to write things like:
The ocean’s current carried the small boat along.
She ducked beneath an awning to avoid the wind’s biting chill.
Wear SPF 30 or stronger to avoid the sun’s damaging UV rays.
The night’s activities had been draining for Elnora.
It is no wonder that many people identify English as one of the most difficult languages to learn. For every rule, there are many exceptions, and there are so many rules to begin with.
I do highly recommend either “A Brief Handbook” or “Essentials of English” as quick and easy-to-use reference books for finding things like this. You can find these at just about any major book store, or at your favorite on-line book retailer.