Copyright 1984 The New York Times Company

By MICHAEL T. KAUFMAN, Special to the New York Times

WARSAW, July 11

Since martial law was imposed in Poland in December 1981, letters smuggled out of prison by Adam Michnik, an adviser of Solidarity, have helped sustain hope in the outlawed movement.

Mr. Michnik is scheduled to go on trial Friday. It is a trial that the authorities tried to avoid by offering him freedom and the right to go abroad. It is a trial that Mr. Michnik demanded.

The smuggled letters by Mr. Michnik, a 38-year-old activist, have been copied by hand, typed, mimeographed or printed in clandestine shops. They have been widely read and discussed.

Mr. Michnik's letters analyze the impasse in which the people and the Government seem unable to move forward or to influence each other. They review what he sees as Solidarity mistakes. At times, they accuse the Roman Catholic Church of timidity in coming to the defense of political prisoners.

Debate From a Prison Cell

From the prison cell he shares with three men convicted of common crimes, Mr. Michnik has continued the debate between state and society, the debate that Gen. Wojciech Jaruzelski, the Polish leader, tried to suppress when he ordered troops into the streets on the night of Dec. 12, 1981.

Most of all, Mr. Michnik has insisted on his day in court to answer charges that he was ''preparing to demolish the system by force.'' Last month, the military court for the Warsaw District set the trial date for him and three others.

One of the co-defendants is Jacek Kuron, 50 years old, who was Mr. Michnik's mentor as well as the leader of his Boy Scout troop. For 20 years Mr. Kuron tried to democratize Poland's institutions, first from within the ruling Communist Party and then from the outside - from jails, factories and Solidarity meetings.