FRANKFURT, West Germany (AP) _ A Lebanese man charged with murdering a U.S. Navy diver during a 1985 hijacking had been put on probation in 1983 after a court ruled he was not likely to break the law again, court evidence indicated.

Mohammed Ali Hamadi, a Lebanese Shiite Moslem, is on trial on charges of murder and air piracy in the June 1985 hijacking of a TWA jetliner on an Athens-to-Rome flight.

U.S. Navy diver Robert Stethem of Waldorf, Md., was shot to death after the hijackers forced the plane to land in Beirut. Thirty-nine Americans were held hostage during the 17-day ordeal.

During a brief hearing Thursday, the court heard a document from a court in the southern West German state of Saarland, which convicted Hamadi of theft and assault.

Hamadi was charged with stealing a cash box from a street vendor and with assault after he smashed it into the face of a man who tried to stop the robbery, the document said.

The court gave Hamadi a suspended nine-month sentence in 1983 after ruling that there was ''no evidence that the defendant is predisposed to further criminal behavior.''

The court said it believed Hamadi, who was ordered to perform 120 hours of community service work, would learn from the experience and not break the law again. Hamadi lived in Saarland from 1982 to 1984.

The document also is being used by the Frankfurt court in an attempt to determine Hamadi's exact age.

The Saarland state court document listed Hamadi's date of birth as June 13, 1964. Hamadi says he was born in 1966.

The ruling on age is important because it will determine the length of any sentence Hamadi receives if convicted. The range is from 15 years to life.

Also Thursday, Chief Judge Heiner Mueckenberger read a statement obtained from Hamadi after he was arrested at Frankfurt airport on Jan. 13, 1987, with liquid explosives in his possession.

According to the statement, Hamadi said he thought the bottles contained alcohol. He said he was to carry the bottles from Beirut to Frankfurt and deliver them to an acquaintance named Ahmed.

Hamadi said he was returning to West Germany to try to set up a car dealership.

His statement to police at the time also listed his birth date as June 13, 1964.

Asked by Mueckenberger if he had anything to say, Hamadi looked at his lawyer and declined to comment.

The trial, which began Tuesday under heavy security in a specially constructed prison courtroom, adjourned after about an hour Thursday because a witness who knew Hamadi during his previous stay in West Germany was unable to attend Thursday's session.

The witness, a social worker, originally had been scheduled to testify July 13 but was later asked by the court to appear Thursday.

The trial's first two sessions have been cut short, raising prospects the proceedings could extend into next year. Officials already have scheduled court days into December and say more could be added.

Tuesday's opening session was suspended after Hamadi refused to participate.

The United States tried unsuccessfully to have Hamadi extradited to stand trial in a U.S. court.

The trial is to resume July 13.

A Duesseldorf court sentenced Hamadi's older brother, Abbas Ali Hamadi, to 13 years in prison in April for kidnapping two West Germans in Beirut in an attempt to gain his brother's freedom.

One hostage later was released, but the second still is being held in Beirut by a radical Shiite group.