10 Grammar Rules Every Teacher Should Know

Rule #9 Stop Apostrophe Anarchy
The apostrophe [ ’ ]serves two functions:

• 
  it marks contractions (don’t, can’t, won’t, didn’t, etc.) and
  
• 
  added with an “s” it identifies possession.
  

Possession is how we refer to the relationship between two nouns, one of which “belongs to” (possesses) the other. The owner-noun is possessive. Examples are:

Nobody’s fault (the fault belongs to no one)

New Year’s Eve (the eve of the new year)
a dog’s bite (the bite of a dog)

Apostrophes NEVER come before “s” to make a noun plural, and they NEVER occur with a verb. Thus:

An onion’s smell is strong.

Onions in the sun smell strongest.
If you smell onions, you cry and your nose runs.

You’ve probably come across words that end with an apostrophe. These are possessive nouns whose last letter is “s” or “z” (like physics, ethics, Charles and Liz) or are nouns that are both plural (with the “s” on the end) and possessive (with an apostrophe). For example:

Physics’ heroes are not well known. (the heroes of physics)

Liz’ bookbag is still here. (the bookbag belongs to Liz)
Students’ parents are welcome to attend. (the parents of students)

It is also acceptable to show possession for nouns ending in “s” or “z” with the usual ‘s added on:

Physics’s heroes are not well known. (the heroes of physics)

Students’s parents are welcome to attend. (the parents of students)

As with all options for rules, you should choose one of the two styles and be consistent in your text.

Short Cut
To determine whether something is possessive or not, try a reverse question. Take the word that follows it and say “whose is it?” The answer word is possessive, and it should get an apostrophe. For example: