Please can you change this into your own words?
Apartheid was a system of legal racial segregation enforced by the National Party government in South Africa between 1948 and 1994, under which the rights of the majority black inhabitants of South Africa were curtailed and minority rule by whites was maintained.
I am inclined to break the sentence up into multiple sentences. I think it allows the reader to better digest the heavy and strong points made in the sentence. It also ensures for you, the writer, that all the needed facts can be transmitted through without being lost. My first sentence would be, "Apartheid was a system of legalized racial segregation in South Africa from 1948 to 1994." This is a nice opening sentence that helps to bring forth the ideas of time frame, purpose, and actual fact. My second sentence would be, "Enforced by the National Party Government, the rights of the majority Black inhabitants of South Africa were curtailed as rule by the minority Whites was maintained." In this sentence, I feel I was able to provide more detail and definition to the first one.
I can see why you would like to rearrange the sentence in your own words. This sentence is almost exclusively in the passive voice. By changing it to the active voice will help with clarity. Here is one way:
Apartheid is a system of legal racial segregation in South Africa. The National Party enforced this policy between 1948 to 1994. This policy curtailed the rights of the majority black inhabitants and maintained the rules of whites.
This definition of Apartheid is actually fairly straightforward. Apartheid is an Afrikaans word that means "apartness" and was originally used in the 1930s and 1940s. This policy of "separate development" became lawful when the Population Registration Act of 1950 was enacted. This law relegated South African citizens into three racial categories: Bantu (black African); white; or Colored (of mixed race). Asians were eventually added as a fourth category. Apartheid restricted non-whites in several ways; land ownership was allowed only in certain areas; social contact between the races was limited, as was educational and public facilities. Further restrictions involved government positions, job placement, and non-white unions. Essentially, Apartheid guaranteed that the minority white population would control South Africa by limiting the rights on non-white citizens.
What you need to do is to take the sentences and simplify them and change them around a bit. Here's how I would do this.
Apartheid was a system in which the various races living in South Africa were kept separate from one another. The system started in 1948 and ended in 1994. During this time, South Africa was ruled by the National Party and that party supported and enforced apartheid. Under apartheid, the black citizens of South Africa, who made up a majority of the population, had their rights taken away. Apartheid helped the whites, who were in the minority, keep political power for themselves.