Rule 1

We can use the apostrophe to show possession. By adding a ‘s’ to a singular noun, one can show possession.

a man’s shoes
the boss’s orders
Mrs. Tony’s car

 A conflicting case in the usage of the apostrophe is when many common nouns and proper nouns end with the letter ‘s’. Eg: Jones or Bus. There are many theories on how to use the apostrophe in such cases. There is no right answer to it, stick with one way of using the apostrophes and remain with it.

Many people just add only an apostrophe following the ‘s’. And some add an apostrophe and another s to every proper noun, be it Toms’s or Jones’s.

Another method is to add an apostrophe for proper nouns and add ‘s’ to the common nouns ending with s.

the boss’s orders

Mr. Harris’ shoes
the bus’s speed
Texas’ weather

One should take extra care while writing apostrophes. It can change the whole meaning of the sentence if the apostrophe is a bit misplaced.

Another way is to use the apostrophe as we speak it. For example, since most people saying “Mr. Harris’s pen” would not pronounce an added s, we would write Mr. Harris’ pen without an added s.  would pronounce an added s in “Jones’s,” so we’d write it as we say it: Mr. Jones’s golf clubs. This method explains the punctuation of for goodness’ sake.

Rule 2

For a regular noun, plurals are made by adding ‘s’ or ‘es’ with the word. Apostrophes have to be used properly in order

Correct: girls‘ night out (girl + s + apostrophe)
Incorrect: girl’s night out (implies only one girl)

Correct: two actresses’ roles (actress + es + apostrophe)
Incorrect: two actress’s roles

Never use an apostrophe with ‘s’ to make a regular noun plural.

Incorrect:  My pant’s are very tight.
Correct: My pants are very tight.

Incorrect: We’ve celebrated many happy Christmas’s.
Correct: We’ve celebrated many happy Christmases.

There are again special cases wherein apostrophes are used to give more clarity to the users signifying that it is a plural that is used in the sentence.

Example: There are some do’s and don’ts.

In the above sentence, without an apostrophe, the reader might think that ‘dos’ is another word altogether. To avoid this confusion, apostrophes are usually used. Not all language experts agree with this practice though it’s widely followed.

With single case letters, it is best that one uses apostrophes.

Example: My u’s look like a’s.

Just imagine the kind of confusion if one does not use apostrophes for this sentence. It will be easily read as “my us look like as”.