Deuteronomy 21 discusses various laws and regulations that were given to the Israelites.

1. Unsolved Murder (Verses 1-9): This section deals with the procedure for atonement in case of an unsolved murder. Life Application: This teaches the importance of community responsibility and seeking reconciliation when there’s unresolved wrongdoing.
2. Marriage to Female Captives (Verses 10-14): This part outlines the laws regarding marrying a woman taken captive in war. Life Application: This can be interpreted as emphasizing humane treatment and rights of individuals even in complex situations like war.
3. Rights of the Firstborn (Verses 15-17): These verses discuss the inheritance rights of the firstborn son. Life Application: It teaches the importance of fairness and justice in family matters, and the necessity to adhere to ethical standards even in personal affairs.
4. Rebellious Son (Verses 18-21): This passage describes how to deal with a rebellious and disobedient son. Life Application: This can be seen as an encouragement for parents to instill discipline and respect in their children, and the need for communities to support the upbringing of young people.
5. Various Laws (Verses 22-23): The final verses of this chapter include laws about the handling of a criminal’s body after execution. Life Application: This might be seen as a call for dignity and respect even in punishment, emphasizing that justice should not be mingled with unnecessary cruelty.

Each of these sections reflects the cultural and historical context of the time. In applying these to modern life, it’s important to consider the underlying principles like justice, responsibility, and respect for human dignity, and to understand how these principles can be applied in contemporary settings.