The status of the mentally ill in Jewish law

The Jewish law goes into great detail discussing the status of the mentally ill. There are many aspects to this question over and above the legal aspects of such a person's rights, obligations, doing business, etc. What is the Halachic approach to a mentally ill person in general? Is this person subject to the code of Jewish law the same as the normal Jew? Should we make an effort to help this type of person fulfil the commandments and prevent him or her from transgressing them or perhaps since such a person is incapable of controlling his or her behaviour, there is no purpose in these efforts? Marriage and divorce are other serious issues to which the Jewish law gives special attention in this context. Marriage must be entered into by a rational and judicious person or the act will not be valid. A very serious problem arises when a husband is mentally ill and due to that halachically cannot divorce his wife and she remains an Agunah. The situation is more complicated as the definition of mentally ill encompasses a broader spectrum of cases. Which psychiatric disorders come under the definition of a mentally ill person who is unable to control his or her behaviour? Which symptoms attest the inability of a person to enter into marriage or to grant a divorce? The Talmud discusses these matters in several places and the Halacha bases its rulings on their conclusions.