The possessive case of nouns is usually formed by adding the Apostrophe (') with the letter (S) before or after a noun:
A) 's is usually added to singular nouns. For example, a cat's tail.
B) ' is added to plural nouns ending in S. For example, two cats' tails.
C) 's is added to plural nouns not ending in S. For example, children's shoes.
D) ' is added to a noun, when the noun ends in S or a sound like S. For example, for goodness' sake.
E) When there are several nouns, ' is used with the last noun Only. For example, the chairman of the committee's report.
Note: The possessive form is usually used in speaking of persons and animals, but not things. For example, we do not say the car's doors; we say the doors of this car. The "Apostrophe S" has another use to show a word is shortened. For example, isn't = is not.