A 20-year-old Naval Academy midshipman was sentenced today to 14 months in prison, forfeiture of pay and dismissal from the Navy after admitting he stole $5,000 in goods from a Naval Academy store.

Clayton Matthew Lewis also admitted to a court-martial here that he was twice absent without leave while being investigated for the seven-month shoplifting spree.

A second midshipman who admitted taking part in some of the shopliftings resigned from the academy in August, academy officials said today, but was not charged after he agreed to testify against Lewis. Academy officials said other midshipmen who received goods from Lewis resigned during the summer, but they were unable to specify how many.

Lewis, the third midshipman to face a court-martial this century, is being confined at the Marine brig at Quantico.

Speaking tearfully as he pleaded for mercy, Lewis, a tall, gangly midshipman from Roanoke Rapids, N.C., said his world "has just fallen to pieces" because of what he had done. "There's no way I could express the remorse I feel for what I have done," he said.

His girlfriend, with whom he ran away from the academy in July, has left him, he said, and with that, "I lost the most perfect and precious thing in my whole life -- a person I'd planned my whole life around. Now that's all gone, too, never to be again."

But Navy Lt. Rebecca Gilchrist, who prosecuted the case, told presiding judge Marine Lt. Col. H.S. Aitkins that "the remorse that he Lewis shows is too late now." She said his thefts were calculated, that he had "corrupted others" and had "dishonored the Naval Academy."

In pronouncing the sentence, Aitkins remarked on the promise of Lewis' early career and described him as a young man who "snatched defeat from the jaws of victory."

Aitkins initially sentenced Lewis to three years in prison, but reduced that to 14 months under the terms of a plea bargain agreement worked out by Lewis, the prosecutor and the Naval Academy.

Lewis could have received up to 11 1/2 years in prison. His sentence will be reviewed by the Naval Academy superintendent and the Judge Advocate General's office in Washington.

Answering questions from Aitkins, Lewis said he started stealing from the midshipman's store at the academy last October, and went into the store between 30 and 50 times to steal. He said he also, with employes' help, took food several times a week from "The Steerage," a cafeteria at the academy.

Lewis said he would simply walk into the Midshipman's Store, pick up the items he wanted, and walk out past the cash registers. He said he was never challenged. Sometimes he would keep the items, which included record albums, sunglasses, clothing and an electric blender, he said, and sometimes he gave them away.

In a written statement prepared for the court-martial, another midshipman, Eric A. Kaiser, 21, of Collingswood, N.J., said Lewis told him he had come up with a "scam" to "screw the academy." Kaiser said Lewis later told him how he stole from the store and the two went to the store together to shoplift. He said Lewis showed him how he kept a running list of everything he stole and its value in the back of a physics composition book.

In another statement prepared for the court-martial, a member of Lewis' company at the academy wrote that his fellow midshipmen were "astonished and angry" to learn Lewis had been stealing.

"He used the academy for his own reasons," Chris Ameswood wrote, "and I don't think he should be allowed to stay in the Navy."

Lewis has paid $2,000 in reparations and Navy investigators recovered many of the goods he stole. Both he and his lawyer, Lt. Christine D. Senseman, declined to be interviewed after the court-martial.