I'm a little confused about making something possessive. For example, if I were to write

America's history is fascinating.
would that be correct, or would "Americas", or "Americas'" be right? Also, suppose you wanted to say that a dog belonged to Mrs. Ness (or anything ending with s). Would you say it's Mrs. Ness's or it's Mrs. Ness' or what? Thanks for any help!

America's history is correct.

This site is excellent for help in using possessives.

http://grammar.ccc.commnet.edu/grammar/possessives.htm

When making something possessive, there are a few rules to follow based on the noun's form or ending.

For singular nouns that do not end in "s," you add an apostrophe and an "s" ('s) to show possession. For example:

- America's history is fascinating.
- The cat's meow is adorable.

When dealing with singular nouns ending in "s," there are different opinions on how to form the possessive. Both forms you mentioned, "Mrs. Ness's" and "Mrs. Ness'," can be considered correct. However, the most widely accepted rule today is to add an apostrophe and an "s" ('s) to singular nouns ending in "s." This is called the "s-apostrophe" rule. For example:

- Mrs. Ness's dog is adorable.

Note that this rule does not only apply to names ending in "s" but to all singular nouns ending in "s."

For plural nouns that do not end in "s," you simply add an apostrophe and an "s" ('s) to indicate possession. For example:

- The children's toys are scattered everywhere.
- The geese's migration pattern is fascinating.

Lastly, for plural nouns ending in "s," you only need to add an apostrophe (') to show possession. For example:

- The dogs' barking woke up the neighbors.

Remember that these rules are general guidelines, and there may be exceptions or disagreements in certain cases. It's always a good idea to check style guides or refer to the specific conventions of your writing or grammar reference.