Complex sentence: The UK is a multi-party system and since the 1920s, the two largest political parties have been the Conservative Party and the Labour Party, before the Labour Party rose in British politics the Liberal Party was the other major political party along with the Conservatives.
Split into simpler sentences: The UK is a multi-party system and since the 1920s, the two largest political parties have been the Conservative Party and the Labour Party. Before the Labour Party rose in British politics the Liberal Party was the other major political party along with the Conservatives.