Rhetoric and Composition/Missing or misplaced apostrophe

For such a little piece of punctuation, the apostrophe is really noticeable when it’s used incorrectly. And, there seems to be a lot of confusion about how it’s used: a casual look at ads, signs, and other everyday writing reveals a wildly exotic sprinkling of apostrophes in all kinds of places. That’s why mastering the apostrophe's uses can really bolster your credibility in writing: so many people get it wrong!

The main thing to know is that apostrophes’ primary jobs are to form possessives and to stand in for missing letters in a contraction. Apostrophes are only very rarely used to form plurals.

Use possessive forms when you want to indicate ownership, or “belonging to.” Possessives are almost always formed by adding an apostrophe and an “s” to the end of a noun (a person, place, or thing). In contrast, plurals are usually formed by adding an “s” or “es” to the end of a noun without an apostrophe.

EXAMPLES
Possessives use apostrophes.
The amendment’s language clarifies the terms left undefined in the original law. “language” belongs to “amendment”; “terms” is plural
A review of the month’s headlines reveals nine front-page pieces about the local school board election. “headlines” belongs to “month”; “headlines” and “pieces” are plural
Sara Jones’ study of language use and class is considered a classic in the field. “study” belongs to “Jones”; the apostrophe moves to the end because the noun ends in “s”

Plurals do not take apostrophes.
Three key ideas emerged in the introduction.
The organization was restructured after decades of poor performance.
All animals have an innate evolutionary drive to pass along genes to offspring.

But plurals that are also possessive do use apostrophes. Notice how the position of the apostrophe moves depending on whether the plural ends with “s” or not.
The book traces the Kennedys’ influence on national politics.
The library science degree offers a special emphasis in children’s literature.
The board changes the policy after the stakeholders’ objections.

Apostrophes are also used to stand in for missing letters in a contraction.

EXAMPLES
The conclusion doesn’t [does not] follow from the evidence.
Remove the test tubes from the sterilizer when the cycle’s [cycle is] finished.
This committee will file a final report when we’re [we are] done with the applications.

Do not use an apostrophe to form the plurals of numbers or acronyms.

EXAMPLES
1980s
eights
three CEOs
these JPEGs