Complex sentence: In 1915 Oklahoma was the last state to append a grandfather clause to its literacy requirement due to Supreme Court cases Guinn v. United States the Supreme Court rules that the clause is in conflict with the 15th Amendment therefore the literacy test was unconstitutional.
Split into simpler sentences: In 1915 Oklahoma was the last state to append a grandfather clause to its literacy requirement due to Supreme Court cases. In Guinn v. United States the Supreme Court rules that the clause is in conflict with the 15th Amendment, therefore the literacy test was unconstitutional.