President Reagan told a nationally televised news conference last night that the "case was closed" as far as Secretary of State Alexander M. Haig Jr. was concerned and declined to give details about the circumstances that led to Haig's resignation last week.

" . . . If I thought that there was something involved in this that the American people needed to know, with regard to their own welfare, then I would be frank with the American people and tell them," Reagan said, when asked whether people deserved to know more about Haig's departure.

Reagan, who was critical of Jimmy Carter for using the slogan "trust me," added that he didn't "think there's anything that in any way would benefit the people to know about the Haig affair or that will in any way affect their good judgment."

Questions at the 36-minute news conference, Reagan's first since May 13, focused on the Haig resignation, the Middle East and other foreign policy issues.

The president denied that Israeli Prime Minister Menachem Begin had promised him in their June 21 meeting in the White House that Israeli forces would not enter Beirut. He said that what deputy White House press secretary Larry Speakes had "called a promise actually was a discussion in which, to be more accurate, the prime minister had said they didn't want to enter Beirut and that they had not wanted to from the beginning."

Last Thursday, when he was asked whether Begin had given Reagan "assurances" that the Israeli army would not enter Beirut, Speakes replied: "We can confirm that these assurances were given in the meeting and certainly we would like to be able to rely on these assurances."

Reagan tried to avoid commenting on U.S. strategy in the Middle East except in general terms, saying that he had to walk "a very narrow line" because of diplomatic efforts to bring about peace in Lebanon.

But he did say that U.S. policy would not be altered by the replacement of Haig with George P. Shultz.

"There's going to be no change in policy," Reagan said, adding that foreign policy "comes from the Oval Office," with the assistance of the secretary of state.

Answering other questions about policy in Lebanon, the president said that the United States had not given Israel "a green light" to go into Beirut but he refused to discuss what the United States would do if the Israelis entered the city.

Reiterating statements that Haig had made during Reagan's recent European trip, the president said that the United States had not been warned or notified beforehand of the Israeli invasion of Lebanon and regretted the bloodshed.

But he also repeated the argument frequently made by Israel's spokesmen and defenders that the invasion was in response to terrorist attacks by the Palestine Liberation Organization across Israel's northern border.

Reagan said that the U.S. goals in Lebanon are to provide guarantees for that border, assistance in creating a strong central government in Lebanon and the withdrawal of all foreign forces, "Syrians, Israelis and the armed PLO" from that war-torn country.

Reagan dealt with two other foreign policy issues that have been a recent focus of his administration--the sanctions against U.S. firms and foreign subsidiaries that prevent them from providing equipment for use on the Soviet natural gas pipeline to Europe, and the sanctions he imposed on Argentina in response to its invasion of the Falkland Islands.

In response to a question as to whether he would lift the sanctions against Argentina, the president said he hadn't yet had a discussion on the matter. He said the United States had done its best to bring about a peaceful settlement of the Falklands crisis.

"It didn't happen," Reagan said. "And there was armed conflict and there has been a victor and a vanquished and now it's hardly the place for us to intervene in that."

His comments closely paralleled those of British Prime Minister Margaret Thatcher after she met with Reagan at the White House last week. She said afterward that she "rather understood" that the sanctions against Argentina would not be lifted.

Reagan was more discoursive on his actions to prevent the supplying of the pipeline equipment to the Soviet Union. Imposition of the sanctions last December and extension of them June 18 was a matter of principle, said Reagan, who also commented that he was aware before he acted that there would be legal objections in Europe to his stand because of prior contracts to deliver the equipment.

Nonetheless, he continued, he would not back away from his position as long as the Soviet government persisted in doing nothing to relax the repression in Poland that caused imposition of the sanctions in the first place.

"I understand that it's a hardship," Reagan said. "We tried to persuade our allies not to go forward with the pipeline for two reasons. One, we think there is a risk that they become industrially dependent on the Soviet Union for energy . . . . The second thing is the Soviet Union is very hard-pressed financially and economically today."

Reagan's response in dealing with the questions about Haig had been worked out in a meeting with his principal advisers earlier in the day. As summed up by Speakes, that strategy is to look "not backward to Haig, but forward to Shultz," which means refusing to give the reasons that Haig resigned or commenting on reports that he was maneuvered into quitting by the White House staff.

But at last night's news conference, the questions were about Haig, not Shultz, and it was evident from their tone that members of the press corps, at least, did not think that the president had explained sufficiently what had happened.