Final answer:
For plural nouns ending in 's', place the apostrophe after the 's'. For plural nouns that do not end in 's', add an apostrophe and 's'. In contractions, apostrophes show omitted letters.
Step-by-step explanation:
When dealing with plural nouns that already end in an s, the apostrophe is placed after the s to indicate possession. For example, in the phrase 'the families' expenses,' the apostrophe comes after the s in 'families' because the noun is plural and ends in s. However, if the plural noun does not end in s, you would add an apostrophe followed by s, as in 'children's game' or 'people's opinion.'
In summary, the placement of the apostrophe in plural nouns depends on whether the plural form of the noun ends with an s. Add an apostrophe and s ('s) to the plural forms that do not end in s, and add an apostrophe (') after the final s for plural nouns that already end in s.
It should be noted that apostrophes are also used in forming contractions, where they indicate omitted letters, such as in the words 'can't' (cannot) or 'it's' (it is).