To form the possessive of a singular noun, use an apostrophe and an s at the end of the word. For example: the child's toys. For nouns that end in s, place the apostrophe at the end of the word. For example: Jesus' parables or Hercules' sword. (Some stylebooks do call for another s in this case: Jesus's parables.)
Use an apostrophe at the end of a noun that is plural. For example: the teachers' lounge or the students' grades.
To express the possessive form of quantities of money or time, use an apostrophe at the end of the word. For example: two hours' drive or fifty cents' worth.
Apostrophes are used to form a contraction. A contraction is a single word that is made up of two words combined by omitting a letter. For example: does not would be written as doesn't.
An apostrophe is used in place of numerals that have been omitted when expressing a year. For example: the '84 election or the winter of '08. The apostrophe is never used when forming the plural of a date, 1940s.