Some of you may know what it's like to read a sentence that has an apostrophe error in it. That some of you may understand how it's so annoying to read a sentence that uses the possessive form of a noun or pronoun when there's no apostrophe needed.
Example: Martin was shocked to see four large bear's sitting at the table.
Bear's is possessive. An apostrophe is added to make the noun possessive. You could talk about a bear who owns an archive full of honey and say that it's the "bear's honey archive." Archive belongs to bear. You could talk about a den that belongs to a bear and call it the "bear's den." Den belongs to bear.
But did the author mean to use "bear's" as a possessive noun or the plural form of a noun?
Remember that a possessive noun is a noun showing ownership or relationship, and is often used as an adjective telling whose.
Martin was shocked to see four bears. In the example sentence above, "bear's" is not showing ownership or relationship. It is not being used as an adjective that tells whose. This is because there isn't any other noun or pronoun in the sentence that belongs to a single bear. There is no ownership or relationship between a bear and another noun or pronoun.
Bear's is supposed to be plural and have no apostrophe, because an apostrophe is not used to make the plural form of a noun.
So, the author (me) should have written "bears," since an "s" (or another small number of letters) is often added for the plural form of a noun.
So the sentence corrected is: Martin was shocked to see four large bears sitting at the table.
(Another note, if we were going to use bear's as a possessive noun, we would actually stick the apostrophe at the end [think Martin was shocked to see four large bears' mates sitting at the table].)
