A Washington Post interview with President Reagan was described incorrectly yesterday as the president's first private White House interview with a newspaper. Reagan granted one such interview earlier, to the Harvard University Crimson, which was published Feb. 6.

Expressing a grim view of U.S. relations with Moscow, President Reagan said in an Oval Office interview that he will refuse to lift the grain embargo at this time and that any summit meeting with the Russians would have to include discussion of "the imperialism of the Soviet Union."

During a 40-minute interview with The Washington Post, reviewing his first two months in office, the president said he entered office wanting to lift the grain embargo, as he repeatedly promised to do during the campaign. But, he said, "I do not see how we could lift it at this time without sending the wrong signal."

In his first private White House interview with a newspaper, Reagan expressed himself on a number of crucial policy questions:

The president has serious doubts about Pentagon plans for land-basing of the MX intercontinental missile because that scheme is "so elaboate, so costly, and I'm not sure that it is necessary or would be effective." He said he supports the idea of the MX missile, intended to protect the United States from Soviet missile attack, but the question of whether the MX should be sea-based, land-based or kept in conventional missile silos is wide open.

Reagan would like to provide U.S. food to aid the hard-pressed Polish people, but such a decision would be contingent upon whether there is a Soviet invasion of Poland or internal suppression of the free trade-union movement.

The new administration's focus of U.S. policy in southern Africa will be to seek a peaceful solution of the Namibian crisis. "We think it begins with an election . . . that just as we did in Zimbabwe should follow the adoption of a constitution that guarantees equal rights to all people in that country, property rights, minority rights," Reagan said.

The president described the government of Angola as a puppet "dominated by the prescence of Cubans, surrogates for the Soviet Union." He said he sympathizes with the anti-Marxist Angolan rebels, but he complained that U.S. ability to aid the Angolan rebels is restricted by the Clark amendment, which the administration wants Congress to repeal.

Reagan gave mixed signals on how he will resolve one of the most important arguments in his young administration: the question of whether to restrict Japanese auto imports. He described himself as a believer in free trade and an opponent of "protectionism," but observed that the Japanese had spoken of observing "voluntary restraints" in their export policies.

Reagan's apparent tilt away from protectionism, despite the recommendation of an administration task force calling for curbing Japanese imports, may have reflected the advice he received last Wednesday at a meeting with prominent economists.

Most of them, including Reagan's favorite economist, Milton Friedman, warned him that any protectionist move by the United States would be likely to start a tariff war with the Japanese and Europe.

The presidential interview, conducted Friday afternoon, reflected Reagan's customary optimism as he discussed the range of problems facing his administration. But the president's mood turned somber on the subjects of Poland and U.S. relations with the Soviet Union.

He said that "any imposition on the freedom of the people of Poland," whether by Soviet invasion or by internal repression, would have a serious impact on Washington's relations with Moscow.

Reagan's message seemed to be that the United States takes an equally dim view of repression of the Polish trade-union movement, whether that comes from a crackdown by the Polish government or from a Soviet invasion.

Answering Poland's appeal for food would be a lot easier, Reagan said, if the Polish government does not "take some drastic militant step against their own people."

The Soviet threat to Poland has played a major role in Reagan's decision to keep the grain embargo in place despite his campaign promises and a "sense-of-the-Senate" resolution calling upon him to lift it.

The embargo, he said, is "something I would dearly love to be able to lift, but the very situation we've been talking about, Poland, the entire international situation is such that I at this moment do not see where we could lift it without sending a wrong signal."

Regan's view of the Soviet invitation to a summit meeting was cool.

"I think it's far too early for that," he said. "I haven't said no, I've just said that's down the road a bit."

Later in the interview Reagan said he has received no indication, other than Soviet President Leonid I. Brezhnev's nine-page invitation to a summit meeting, that the Russians are willing to give up anything in exchange for peaceful relations with the United States.

"I have made it plain that such talks when they do take place should involve not just limiting ourselves to arms reductions or theater forces and so forth," Reagan said.

"I think the whole matter of the imperialism of the Soviet Union, their expansionism, must be a matter for discussion. Are they going to continue . . . exploiting where there are differences and where there is trouble? And are they going to continue this massive buildup of weaponry that is the greatest that any nation has ever made in all the world?

"Or are they willing to sit down and talk about how we can eliminate the difference, reduce the threshold of danger from strategic weapons, respect the right of people to self-determination in their countries and so forth?"

Reagan's questioning of the present proposal to scatter 200 MX missiles on a drag-strip system through the Neveda and Utah deserts is a blow to Pentagon and congressional hardliners who have argued that present national security estimates are dependent on this basing system.

It indicates that the president shares the skepticism expressed by Secretary of Defense Caspar W. Weinberger, who argues that environmental lawsuits could delay land deployment of the MX system beyond 1986. Weinberger has named a panel of experts to report to him by June 1 on the best solution.

A number of western politicians in the region of the country where Reagan has the strongest support have objected to the desert deployment of the MX. But Reagan's questioning of the land-basing system goes beyond the rangeland it would require.

"It's not only that," he said. "It's so elaborate, so costly, and I'm not sure that it is necessary or would be effective. It's again an indication of this whole effort such as in the SALT talks to have verifiability so you can create a great, elaborate, costly system in which you can hide the missile except that the enemy has to know the missile is there. And it doesn't make much sense to me."

The president was asked if his reply means that MX sea-basing is under consideration.

"I think there are any number of [options] ranging all the way from silos such as we presently have," Reagan replied. "Silo, sea-based, they're all being looked at."

Concerning draft and registration, the president said that removal of draft registration is a low-priority item for his administration, but that he remains opposed to a draft.

"I think the most important thing we've been concerned with right now, he said, "is making the voluntary military more effective and meeting some of the real problems that they have."