Eighteen-year-old Arthur (Tiger) Robinson, a graduate of the Sidwell Friends School who is going to Dartmouth College this fall, asked the first questions of the candidates for mayor at the Jack and Jill candidates' forum Wednesday night.

He asked Morris Harper how voters could trust a man who misled them when reporting campaign contributions. He asked Charlene Drew Jarvis if she was in the race merely "to siphon off votes from Patricia Harris for Marion Barry."

He asked John Ray how he could run for mayor with so many people saying that his record on the City Council is so bad, and, after saying that incumbent Marion Barry had failed to significantly improve city services as he had pledged in 1978, he asked Barry what promises he'd make this time to get reelected.

Finally, Robinson wanted to know why lawyer Patricia Roberts Harris thought she should be mayor if she hasn't been involved in local affairs.

The questions prompted a flurry of barbed one-liners, the sharpest exchange yet among candidates for mayor, remarks that became even more nasty at points because the audience was dominated by supporters of one candidate or another egging the rivals on and cheering the best put-downs.

It had all the trappings of a well-polished political version of oneupmanship before an audience of middle-class blacks valued by the candidates because they are some of the most dependable Democratic voters in the city. It was a night when the discussion of what many would consider substantive issues appeared to come only in passing.

Jack and Jill is one of the oldest back social and civic organizations in the nation and a key part of Washington's black establishment.

The responses to Robinson's first set of questions came in a series of slow, cautious answers.

Harper did a double-take at the tough query about the fact that more than half of the campaign donations he reported were only pledges and not funds received. He said his problems came about because the six words a canidate hears most from donors are "the check is in the mail" and he had been "foolish enough to trust them."

Jarvis, the only one of the candidates who is a member of Jack and Jill, said she was in the race to win, not to block anyone and would beat Harris while "Mr. Barry will lose on his own accord."

Ray, who grimaced and dropped his head at the question about his council record, responded that he had worked with another council member on a condominium conversion bill, had been active in fighting crime and had been involved with the city since he came here 15 years ago.

Barry took the edge off his question by first asking the audience of about 110 persons at the University of the District of Columbia to applaud Robinson for asking such good questions. Then he said he was "not a perfect mayor, but a good mayor."

And Harris responed to questions about her local involvement by saying she was involved in the civil rights movement in Washington "when Mr. Barry was still in Itta Bena" and was working for home rule when "he was still walking around the streets trying to find what he wanted to do."

She didn't forget Ray, asking him, rhetorically, if she wasn't on the bar committee that graded his bar exam entitling him to be a lawyer here.

But Robinson had other quesions. He asked each candidate to reveal his weakness.

Jarvis said hers was fund-raising, but then added that she wasn't giving out contracts and jobs to raise $700,000, which Barry has raised.

"I don't need $700,000 to restructure my image," she said. Jarvis said even without money, she'd be in the race at the end. "Unlike some other candidate here tonight," she said, glancing at Ray.

Ray came back by saying his major weakness was determination. He told Jarvis that she had never fought a bobcat like him. "Miss Jarvis," he said, politely, "I'll be here when you're gone."

Harris said her weakness was demanding good performance from people. "If you want a mayor who only starts to look like he's doing his job in the last year of his administration, then you should reelect Mr. Barry," she said. "If you want four years of good government, you should vote for me."

Barry said the only way to tell if any of the candidates was telling the truth was to find out what they were doing when they weren't the mayor. One candidate, he said as he looked toward Harris, hadn't been in Shepherd School, Harris' neighborhood school, in seven or eight years.

Later Harris struck back when one of Barry's supporters asked why the challengers had to be critical of the mayor: "I've said all along he's done the best he can," Harris said. "That's what so sad."

Publisher Dennis Sobin was the only candidate who stayed above the fray. "I'm not going to insult the audience's intelligence here tonight by giving them one of my strengths disguised as one of my weaknesses," he said.