In his second determined show of optimism in two days, President Carter said yesterday that the nation has "turned the corner" on inflation and can look forward to a significant decline in the inflation rate this summer.

The president made this rosy assessment, which would mark a decided change in the generally gloomy economic news of the last several months, during a White House meeting with executives from major accounting firms.

"We believe that we have turned the corner," he said. "We're making good progress on interest rates coming down and we have sound indications that the inflation rate will also drop significantly during the summer.

"The balanced budget, which will be the first in 12 years, is well under way," Carter continued. "We have excellent cooperation from the Federal Reserve on restraining consumer credit and other matters that they have initiated."

The president's decidedly upbeat assessment is apparently the message he intends to take to the country, still reeling from the failed attempt to rescue the American hostages in Tehran, when he resumes traveling late next week.

Carter made equally optimistic comments about the country's strength and its future to a group of civic leaders Wednesday in what appeared to be a determined effort to project an aura of confidence after so many months of frustration and setbacks, the most serious of which was the aborted rescue attempt.

Referring to his decision to resume travel and campaigning around the country, the president told the accountants:

"I will be moving around the country more in the next few weeks, primarily to rally the American people to support the domestic and foreign policies that are so important to us and which are so intensely and intimately interrelated."

White House press secretary Jody Powell said Carter's first trip will be late next week, and that it will be official presidential travel to a state that has already selected its delegates to the Democratic National Convention. He said events in at least two states were being considered, but that no decisions had been made.

Officials said a "town meeting" in Philadelphia, one of the many events that had been scheduled but was canceled after the Nov. 4 takeover of the U.S. Embassy in Tehran, was a possibility. They said the president is also being urged by some to give an economic speech in Detroit, where the automobile industry is feeling some of the severest effects of the recession.

In addition, Carter has promised voters in Iowa, where he won his first election contest with Sen. Edward M. Kennedy (D-Mass.) in January, that he would return to that state to thank them as soon as he could.

On Wednesday, the administration was hit with new economic indicators showing a fast-developing recession, and Carter's anti-inflation chief, Alfred E. Kahn, predicted that unemployment could hit 8 percent in early 1981.

Asked about this yesterday, Powell said that the president's "turned the corner" remark applied to inflation and not to the economy overall.

He said the administration has been predicting an economic slowdown this year, "but always made clear that we consider inflation at this level to be our No. 1 problem."

Once inflation is "down to manageable level," Powell said, it may be possible to stimulate the economy through tax measures. However, he made no predictions on when this might occur.

Powell also continued to defend Carter's statement Wednesday that the Iranian crisis and other problems are now "manageable enough" to allow him to resume travel and campaigning.

He said Iran "continues to be a pressing issue, but in terms of the president's time, it requires somewhat less than during the last three weeks leading up to the rescue attempt and during the period of very active diplomtic discussions."

Powell said he knew of no new diplomatic initiatives to free the hostages.

Sen. Henry M. Jackson (D-Wash.) told reporters yesterday that the president should have dropped his Rose Garden strategy a long time ago. Jackson said it has been a grave mistake to allow the hostage situation to become the overwhelming focus of U.S. foreign policy.

Jackson said he hopes the new secretary of state "will get us off this wickett of hostages morning, noon and night." He said the extraordinary emotion whipped up here by the situation plays into the hands of the militants and the Kremlin. It does irreparable harm to U.S. foreign policy, forcing it into a very narrow focus, and provides a cover for Moscow to quietly take over Iran from within, he said.