Complex sentence: Many teachers joined the informal economy, or black sector, during the economic crisis by participating in cross-border trading with Botswana and South Africa because civil servants were not required to have visas at the time.
Split into simpler sentences: Many teachers joined the informal economy, or black sector, during the economic crisis. They participated in cross-border trading with Botswana and South Africa because civil servants were not required to have visas at the time.