Another are that some authors seem to have trouble is in telling the differences between when words are plural and when words are possessive, or showing ownership.

Here's a simple guide:

Its is the possessive form of "it". "Its" is used as a modifier before a noun. ("The airline cancelled its early flight to New York.") This possessive case is often misused and confused with "it's", which is a contraction for "it is".

One's indicates possession or ownership by a single individual. ("One must have control over one's temper.") The word "ones" (without an apostrophe) may be used to indicate a group of serveral, unique individuals or objects, but could likely be replaced with a better descriptive word.

"His and "hers" are both possessive words used to indicate the one or ones belonging to him or her. The use of the term "her's" is grammatically incorrect.

Theirs is the possessive term used to indicate the one or ones belonging to them. The word "their's" is grammatically incorrect. "Their's" is often confused with "there's" which is a contraction for the words "there is".

Ours is also a possessive term. Again, the term "our's" is incorrect.

Yours is also possessive. The term "your's" is incorrect.

"Why's", "How's", "Where's" etc. are contractions used to indicate the phrases "why is", "how is", "where is", etc. If an author wishes to show plural forms of these words, the apostrophe should be removed; as when the author writes "The whys and hows", for instance.

Mine is a possessive pronoun all on its own ("That book is mine.") "Mines" are either holes dug into the grouond to extract ore, metal objects floating slightly below the surface of a body of water and filled with explosives to sink ships, or they are small explosive charges buried slightly underground to eliminate footsoldiers or to disable armor or support vehicles. The word "mine's" is a contraction used to represent the words "mine is" ("Mine is bigger than yours.")