Complex sentence: Marius is shocked, and at first attempts to persuade him to stay, but reluctantly accepts Valjean's decision to leave, promising not to tell Cosette that her father is a fugitive.
Split into simpler sentences: Marius is shocked, and at first attempts to persuade him to stay, but reluctantly accepts Valjean's decision to leave. He vows that he will not tell Cosette that her father is a fugitive.