Use an apostrophe to show that a noun is possessive. Possessive nouns are nouns that show ownership. For example, you would refer to "Bob’s hat" or "Sue’s desk," with the hat belonging to Bob and the desk belonging to Sue. Sometimes, however, the ownership is implied and a little harder to see, such as a day’s work (with the work belonging to the day) or the table’s legs (the legs belonging to the table). When you are unsure, try turning it into a phrase.
Table’s legs = the legs of the table
A day’s work = the work of a day
To show possession for a singular noun: Add an apostrophe [‘] and s.
When to add –‘s: If a noun does not end in –s, add –‘s.
Susan pruned the tree’s branches last week. Thank you for finding the children’s bikes.
When to add only an apostrophe: If the noun is plural and already ends in –s, add just an apostrophe.
Both students’ papers were well-written. Both cars’ tires were flat.
How to show joint possession: To show that two or more nouns both have possession of the same thing, use –‘s or –s' with the last noun only.
Have you seen Trisha and Greg’s new car, yet?
Mary and James’ tickets came in the mail today.
How to show individual possession: To show two different people each owning something separately, make all nouns possessive.
Mary’s and Michael’s grades have been posted.
Use an apostrophe and –s to show that an indefinite pronoun is possessive.
Someone’s book has fallen into the snow.
Use an apostrophe to show omissions in contractions and numbers. In contractions, apostrophes take the place of the missing letters. For example, "it’s" stands for "it is," and "can’t" stands for "can not."
It’s terrible that James can’t find his textbooks he lost last week.
Note that "it’s," with an apostrophe, is not possessive—"it’s" stands for "it is." When you want the possessive pronoun, you want to use "its," with no apostrophe. If you are unsure whether you want to use "it’s" or "its," say it as "it is." If it does not make sense as "it is," then you want the possessive pronoun "its."
In addition, an apostrophe can be used to show the omission of the first two digits of a year. For example, ’95 stands for 1995.
A similar situation happened in ’95.
Note that when discussing something that happened during a particular time period, such as “during the 1990s,” no apostrophe is needed.
An apostrophe is no longer used in plural numbers, plural letters, plural abbreviations, and words mentioned in a sentence as words. Here are the correct forms to follow:
She skated several figure 8s in a row. Two large Ms were painted on both doors. He collected two IOUs today. I must have told him twenty nos today!