To indicate possession:
> The cat's meow is loud.
> The woman's bag is on the table.
> Mary's book is missing.
To indicate omission of letters or numbers:
> I'm going to the store. (I am going to the store.)
> Don't forget to pay the bill. (Do not forget to pay the bill.)
> The year '68 was a turbulent time. (The year 1968 was a turbulent time.)
To indicate the plural of abbreviations or acronyms:
> The UN's decision was controversial. (The United Nations' decision was controversial.)
> The GOP's candidate won the election. (The Republican Party's candidate won the election.)
To form certain verb tenses:
> She's working on a new project. (She is working on a new project.)
> He'd already finished the report. (He had already finished the report.)
In contractions:
> I can't believe it. (I cannot believe it.)
> You're not going to like it. (You are not going to like it.)
> They'll be here soon. (They will be here soon.)
To indicate certain sounds or words:
> The baby's "ba-ba" sound is so cute.
> The word "apostrophe" is often misused.
These are just a few examples of how the apostrophe is used in English.