Poll taxes, literacy tests, and grandfather clauses were used in the south after 1890 to
(1)support the goals of the freedmen’s bureau
(2)deny suffrage rights to african americans
(3)undermine the “separate but equal” ruling of the supreme court
(4)enforce the amendments enacted during the civil war and reconstruction

The Grandfather Clauses, poll taxes, and the literacy tests that were used in the Southern states after 1890 were used to deny the suffrage rights to the African Americans. The grandfather clause only allowed people to vote who had voted previously. Since the African Americans had never voted, this made it impossible for them to vote. Since the African American people were slaves previously and had no formal schools, they couldn't pass the literacy tests. Also, they couldn't afford to pay the poll taxes. This all changed with the Civil Rights Act in 1964. 

They made a checks and balance system to equal power within the branches. the "check" meant that one branch would watch one branch and "balance" them if they were too powerful. if you want more info on this, i would just google checks and balance system.