Complex sentence: Tens of thousands of African Americans registered to vote after the end of white primaries, but many were still excluded from voting as states used other discriminatory practices, including poll taxes and literacy tests (administered subjectively by white registrars) to keep African Americans from voting.
Split into simpler sentences: Tens of thousands of African Americans registered to vote after the end of white primaries. But, many were still excluded from voting as states used other discriminatory practices, including poll taxes and literacy tests (administered subjectively by white registrars) to keep African Americans from voting.