Apostrophes are sometimes used to replace missing letters in contractions such as don't (do not), couldn't (could not), or that's (that is).
In formal college writing, it is often best to avoid contractions (which are considered informal) and simply write out the words.
The waiter shouldn't have poured an entire pot of coffee over my head just because I didn't order dessert; that's for sure.
The waiter should not have poured an entire pot of coffee over my head just because I did not order dessert; that is for sure.
Note: Contractions can be used when you are writing dialogue between two people/characters. Because people actually speak in contractions, it makes sense to use them when trying to create a realistic-sounding conversation.
Apostrophes are used to show possession.
Add an apostrophe and an "s" to a noun to show that the noun owns something.
If Bob owns a car, we call it Bob's car.
If Bob's car has a stereo system, we call it the car's stereo system.
If the noun already ends in "s," just add an apostrophe to show that the noun owns something.
Some nouns just happen to end in "s" (for example, "Mr. Jones"). Thus, we would write, "Mr. Jones' house is on fire."
Some nouns end in "s" because they are plural (for example, "the boys") Thus, we would write, "The boys' bicycles were parked by the tree."
When writing the possessive form of a pronoun (such as I, you, he, she, it, we, or they), an apostrophe is NOT needed.
The house lost it's roof in the storm.
I think that last cupcake is her's.
My car is parked next to their's.
The house lost its roof in the storm.
I think that last cupcake is hers.
My car is parked next to theirs.
Be careful to only use apostrophes when there is possession (someone/something owns something)
The firefighter's often eat breakfast at Bill's diner.
Do the firefighter's own anything? No. Therefore, no apostrophe is needed.
Does Bill own anything? Yes--the diner. Therefore, that apostrophe can stay.
The firefighters often eat breakfast at Bill's diner.