How and why had blacks, particularly in the south, been subjected to second-class citizenship by 1900? why were jim crow laws predominantly in the south? how did jim crow happen?

They were segregated and forbidden from doing numerous things because of the jim crow laws. These laws established where they would be able to live and where not, where they would be able to go to school and where not, and in what manner would be they allowed to vote, which usually resulted with them not voting. They was a systematic racism scheme in the south to enable this.

They were found in the south because that's where slavery had been a dominating force and in response to the reconstruction era, the white racist governments from the south imposed numerous laws. Although they couldn't enslave african-american people, they would remove them from all spheres of society completely.

Jim Crow was a character made by white people which showed an african-american person who was depicted as a racial stereotype caricature which spread fast and reinforced the idea that black people were either stupid or are tricksters or are malicious or similar things. The set of laws that reinforced these ideas then was called Jim Crow Laws because of the caricature.

Blacks had been subject to second-class citizenship by 1900 thanks to the development of Black Codes and Jim Crow Laws. These laws limited the rights guaranteed to black citizens in the US Constitution.

For example, many southern states implemented literacy tests, poll taxes, and the grandfather clause. All three of these obstacles to voting targeted black citizens as their previous status as slaves ensured that they had little/no money, had no formal education, and did not have ancestors who had the ability to vote.

These types of laws were particularly popular in the South due to the fear of blacks taking over the Southern governments. Considering the fact that many blacks were slaves for centuries before 1865, many whites felt that blacks were the inferior race and that they did not deserve the same power/rights as white citizens.