Testifying softly that she has been trying to "forget the bad," Marina Oswald said yesterday that she lied to the FBI and the Secret Service after the assassination of President Kennedy because she was afraid of being prosecuted herself.

But even now, 15 years later, she refused to say whether she thought her late husband, Lee Harvey Oswald, killed the president. Oswald, she indicated to the House Assassinations Committee, was still a mystery to her, a man she could never understand.

"I know one side of Lee," she testified near the end of a day-long hearing, "but it would take a judgment of his whole character to put all of the pieces together. I don't think I am qualified to do that."

Flanked by four federal marshals, she identified herself to the committee yesterday morning as "Marina Porter," a quiet suburban housewife who lives near Dallas with Kenneth Porter, who she married in 1965.

A plainly reluctant witness at times, she professed to have forgotten much about the day of the assassination and the night before, when Oswald came home for dinner for the last time.

But she said that she had realized months earlier, when he tried to assassinate Gen. Edwin Walker, now retired from the Army, in April 1963, that Oswald was quite capable of killing someone.

"All of a sudden, I realized it wasn't just a manly hobby [he had] of possessing a rifle," she said. "It seemed like he was capable of killing someone with it. I was very disappointed. I had no place to go."

"Why didn't you go to the police?" committee counsel James McDonald asked.

"I should have but I didn't," Marina Porter replied. "I guess whatever sense of loyalty was in me . . .," she added, than halted before finishing the thought. "I could not speak English, anyway," she concluded.

She also made clear that she was afraid of her first husband. At one point, Rep. Richardson Preyer (D-N.C.), asked her whether it had ever occurred to her to dispose of the Mannlicher-Carcano rifle and the pistol Oswald had bought, especially when they are living apart, he in New Orleans after the Walker incident and she in Irving, Tex., with a friend, Ruth Paine. The rifle was stored in Paines' garage.

Porter said she never really entertained the notion. "Most of the time I try not to do something that would antagonize him," she told Preyer. "He was quite brutal sometimes toward me."

"What do you think he'd have done if you had gotten rid of the rifle?" Preyer wondered.

"He'd probably get rid of me," she replied with a nervous laugh, then paused. "I cannot say what he would have done," she added.

Smartly dressed in a tan skirt and striped brown blouse, Porter, now 37, depicted herself in the harrowing days after the assassination as a frightened young woman distracted by fears of what might happen to her and mortified by what had happened in Dallas. She said she wanted to protect her husband, too, and she quite frankly developed a quick distaste for the FBI.

"The FBI was sometimes a little bit too brutal," she protested. "In some ways I wanted to punish them by giving incorrect information."

Accordingly, Porter said, she did not tell the FBI of Oswald's trip to the Cuban embassy in Mexico City in September 1963.She denied seeing ammunition around the houses and apartments where they lived. She even lied to the Secret Service, although "they were the nice guys," when she told them that Oswald had never used the alias of Alek Hidell.

"I thought if I told them about Mexico (City), I would be as responsible as he was for what he did," she told the committee. "I was trying to protect myself as well." As for misleading the Secret Service, she said, "my emotional conditions weren't quite normal."

Despite all that, Porter insisted, "when I gave testimony to the Warren Commission, it was the truth."

The committee's questioning was markedly gentle and uncritical. At one point, National Public Radio reporter Nina Totenberg told her audience that the panel was handling Marina Porter "with kid gloves." Moments later, chief committee counsel G. Robert Blakey handed the reporter a note. It said: "Do you want us to beat up on a widow?"

"In fairness, he was teasing," Totenberg said later.

Warren Commission critics have charged the FBI and the Secret Service put pressure on Marina Oswald to incriminate her husband and thus led her to change her testimony on various points.

She denied that she was ever pressured to give any evidence which she knew to be false. She said she misled them at first because of "human mistakes, human error in my character.

"It was not because they tried to twist my arm and told me what to tell," she said.