Question

what is wrong with the sentence "The modern American family differs in many significant ways from their nineteenth-century counterpart."?

Answer 1

What is wrong in this sentence is the pronoun their.
As you can see, the pronoun their in this sentence refers to the modern American family. Since the word family is singular, the pronoun which modifies it also has to be singular. So, instead of using the pronoun their, it should be its.