Soviet leader Mikhail Gorbachev quickly concluded in his first private session with President Reagan at the Geneva summit that any compromise on Reagan's Star Wars space defense plan was unattainable at this meeting but that a useful dialogue with the American leader could be established and perhaps deepened, according to Soviet sources.

Gorbachev found the one-on-one conversational format initiated by Reagan to fit his own purposes of testing Reagan's mettle, and the Soviet leader decided early in their meeting to stay with that same intimate format the following day when he was the nominal host of the talks, according to these sources, who were in Geneva during the summit.

That decision, and the way in which Gorbachev conducted a forceful counterattack on Reagan's position on the space weapons system when the two leaders were joined by their aides, have demonstrated how firmly in control Gorbachev is at this point and his apparent lack of concern with appearing to be part of a collective leadership, these accounts suggest.

The unscheduledprivate sessions with Reagan persuaded the Soviet leader that superpower relations could be improved, the Soviet sources report, but they also provided him with a sense of where the sharpest divisions lie.

Gorbachev, in his televised speech in Geneva Thursday, said the private sessions were "sharp" but "productive" and of "crucial importance" at the summit.

At the same time, the private disputes between the two leaders over the plan popularly known as Star Wars, but officially called the Strategic Defense Initiative, reportedly gave Gorbachev the full jolt of Reagan's hardened commitment to the space-based weapons system.

Gorbachev made his hardest pitch against SDI, including a sharp argument that it is destabilizing and a threat to world security, in private, according to Soviet officials.

"Gorbachev knew what he wanted to do during the summit," observed Georgi Arbatov, director of the Soviet Union's official Institute for the Study of the U.S.A. and Canada, "and he really didn't need advisers or onlookers around to cheerlead."

"It's easier for him to speak out out of the presence of his advisers," a Soviet source in Geneva said. "Even if they don't say anything, their presence is inhibiting."

Soviet sources indicated that in his speech Thursday Gorbachev gave a detailed account of his most intense give and take with Reagan, a conversation he described as "very sharp indeed."

The Soviet leader restated the arguments that he made against SDI, the various disarmament agreements the United States could achieve if it rejects SDI, and the countermeasures it would face if it continued with the program.

Gorbachev even recounted his hard replies to Reagan's pro-SDI position: "You're not talking to simple folk, and if you stick by your position, we'll do something about it."

At another point he recalls telling Reagan, "You don't believe us . . . . why should we believe you any more than you believe us?"

Nevertheless, Reagan made "a good impression in personal terms" on the Soviet side, one senior Soviet official said, and that helped move the talks forward and in a positive way.

"We found him a good listener," one Soviet official said of the U.S. president.

But impressions of his alertness were mixed. One official reported that Reagan "seemed quite fit and alert despite his operation last summer for cancer ," while another said that "in general, he seemed to lag behind the Soviet leader in conversations."

On several issues, the image Gorbachev projected to Reagan administration officials privately and to black political leader Jesse Jackson in a meeting between the two clashed with the image beamed back into the Soviet Union from Geneva during the summit meetings.

In off-the-record chats with Reagan administration officials in Geneva, Gorbachev apparently talked freely about his will for a political solution in Afghanistan. But in his one-hour-40-minute report on the summit after his final appearance with Reagan, a show beamed live to the Soviet Union, Gorbachev didn't mention Afghanistan at all.

Nor did he refer to conversations with Reagan on human rights, which U.S. officials said were "extensive."

Although he spoke at length with Jackson about the accomplishment of Jews in the Soviet Union, the seven-minute report of the meeting broadcast on Soviet television did not include those remarks. Instead, the news clip showed Gorbachev talking about arms control.

Despite the apparent contradictions, the private approach to interlocutors seems to suit Gorbachev's style.

Making his case directly and away from his circle of advisers typifies Gorbachev's confidence in his persuasive qualities, western Kremlinologists say. "He believes he can convince anyone of the rightness of his position," said one western diplomat in Moscow who has met the Soviet leader.

The private discussions on sensitive areas that Gorbachev held with Reagan follow a pattern that he has set in conversations with French President Francois Mitterrand and Indian Prime Minister Rajiv Gandhi. He has met each leader with only interpreters present.

Soviet sources declined to say how much Gorbachev relied on his team of six advisers who joined in the full sessions when he was not in meetings with Reagan.

But unlike the late Leonid Brezhnev, Gorbachev rarely appears in pictures surrounded by his advisers. "He commands a loyalty in those who work for him rather than the other way around," one official said. "And there is no doubt about who makes final decisions."