Complex sentence: Prevention of schizophrenia is regarded as difficult because there is no reliable markers for the later development of the disease, and much of the evidence for the effectiveness of early interventions to prevent schizophrenia is inconclusive.
Split into simpler sentences: Prevention of schizophrenia is difficult as there are no reliable markers for the later development of the disease. The evidence for the effectiveness of early interventions to prevent schizophrenia is inconclusive.