Salvadoran President Alvaro Magana said yesterday that leftist guerrillas in his country can be defeated next year if U.S. aid continues and there is no increase in aid to the rebels from Nicaragua. On the other hand, he said, there could be Marxist rule in all Central America within a year if the guerrillas take over his government.

Magana, who spent three days in Washington pressing his case to members of Congress and President Reagan, made those remarks in separate interviews with two small groups of journalists. He added in a third interview that he would rather forego U.S. aid than be forced to hold unconditional talks with the guerrillas.

Several members of Congress have proposed making such talks a condition of further U.S. military aid. "We are not going to satisfy it," Magana said. "There is no alternative. We have some principles."

Magana, 57, said that Salvadoran troops are better trained and have more logistical support than previously. The Army has had more reenlistments with new incentive pay and can now hold more territory, he said. "Very likely it will be finished next year," he said, adding, "but if there's more Nicaraguan aid, I could be wrong."

Magana told another group of reporters that Reagan was correct in assuming that the fall of El Salvador to the guerrillas would soon extend the war to all Central America. "They'd be getting close to the United States then because only Mexico is in between," he said.

Magana was careful in his meetings with the news media yesterday to stress that while he is willing, even eager, for talks to begin with the guerrillas, the only subject on the agenda will be the terms for the left's participation in elections. The Salvadoran peace commission has real power to negotiate, he said, but only on that subject.

Ruben Zamora, North American spokesman for the combined directorate of the Farabundo Marti National Liberation Front and the Democratic Revolutionary Front (FMLN/FDR), released a statement Friday asking the status of the peace commission.

Magana said that he had not seen that statement.

Zamora also offered to negotiate directly with the United States as one of the parties involved in the Salvadoran conflict.

Magana rejected that idea. "The United States is not a belligerent," he said.

He added, however, that he has no objection to the participation of the president's special envoy, former Florida senator Richard B. Stone, in talks on the elections. "He's trying to help," Magana said, adding with a laugh, "and in any event he isn't going to ask me."

Magana repeated his assertions that a jury trial should begin within eight weeks in the case of five men charged in the December, 1980, murder of four U.S. churchwomen in El Salvador. For U.S. aid to El Salvador to continue, Reagan must certify to Congress by July 22 that progress is being made in that case.

"People have the impression that the delays are on purpose. That is not the case . . . . We have a different system," he said.

Magana said that since January the military high command has been discussing the possibility of naming a field commander to take charge of the war effort but that nothing has yet been decided.

He said that he remains confident that elections planned for the end of this year will take place, possibly in two stages in November and December, but that the final decision rests with the General Assembly in San Salvador. A draft of the proposed constitution outlining election procedures will be completed next week, he said.

Asked whether he had made progress in wooing Congress, Magana said that everyone had been so polite it was hard to tell. "The first person that asked me that question was President Reagan," he said.