After two difficult years in the White House, patches of gray have begun to show in the once black-and-white world of Ronald Reagan.

The man who at the advent of his presidency described Soviet leaders as "liars" and "cheats" now talks about the Russians in human terms, saying they, like western peoples, are in a precarious economic position and have incentives to "rejoin the family of nations."

The president, who came into office convinced that the nation's economic woes could be easily resolved and the federal government's budget quickly balanced, now recognizes that his administration faces a long and uncertain period of grappling with problems that appear less simple than they did in 1981.

This was the dominant impression that emerged from a wide-ranging and reflective Reagan interview with The Washington Post last Thursday in the Oval Office.

Without yielding essential ground on his fundamental convictions, Reagan displayed in this 38-minute conversation a sense of nuance that suggests he is responding to complex events with something other than the stark and simple approach that has characterized his political career.

Asked what he thought the long-term future of the Soviet Union would be, Reagan replied that recession was part of a "worldwide pattern" that embraced both capitalist and socialist countries.

" . . . That pattern goes beyond the Iron Curtain. Probably not because of the relationship with any of the western world, but because of their own emphasis on rearmament making it impossible for them to meet the consumer demands of their people," Reagan said. "The Soviet Union, we know, is in a really precarious economic position. My own feeling is that this may offer a great opportunity for us if we could convince them that there was a way for them to rejoin the family of nations . . . . "

Asked about the progress he has made toward his cherished goal of economic recovery through reduction in the size of government, Reagan acknowledged that even with all he has done there is "a limit as to what we could do . . . " to cut the federal budget during a period of economic stagnation.

"You can't really cut the budget enough to balance the budget," Reagan said. "You cannot raise taxes enough to balance the budget. The answer to balancing the budget is restoring the economy . . . . That is what will end the deficits, by increasing the gross national product in proportion to the amount of money the government is spending."

After the interview, some of Reagan's closest associates warned not to interpret these statements as a sign that the president was retreating from his basic aims. They pointed out, as Reagan himself indicated during the interview, that the president remains skeptical of Soviet intentions abroad and committed to the purposes of his economic program at home, especially income tax reduction.

But Reagan's aides also believe he is approaching a crucial period of his presidency. Some of them said Reagan is feeling the accumulated pressures arising from his inability to produce a quick economic recovery, solve intractable foreign conflicts or convince an increasingly recalcitrant Congress that he is leading the nation in the right direction.

After two years, Reagan appears to have a glimmer that time is running out on his ambitious goals.

In the interview, Reagan demonstrated a more secure grasp of issues, especially in foreign policy, and a greater appreciation of the subtleties involved in reaching his larger goals than he did in a similar session with The Washington Post on March 27, 1981, three days before he was severely wounded in an assassination attempt. Reagan took five months to recover from that shooting, which, according to aides, seriously delayed his progress in mastering the knowledge needed to be an effective president.

"I see a lot more confidence in himself and his own judgments than he had when he became president," said deputy chief of staff Michael K. Deaver, the aide generally regarded as closest to the president. "By nature he is a cautious man. I see him now as following his instincts much more because he has a hold of presidency and he has the information system down . . . He is comfortable in the job even though it's tougher and longer work and harder hours than it was in the beginning."

Since childhood, Reagan has been an irrepressible optimist and this personal quality has come to the fore as his problems have mounted in the White House.

Asked to predict where he thinks his presidency will be a year from now, Reagan looked forward to a strong economic recovery and reduced unemployment. " . . . As the economy takes off, I think that we'll be seeing that what we've done works," Reagan said.

At another point, he was asked whether the legacy of his administration would permit another Republican presidential candidate to run and win in 1984.

"I certainly would hope so, and I believe this," Reagan replied. "I believe that by that time we're going to see that the program that we put in place does offer what we promised and that is a solid recovery, a lasting recovery."

Reagan did not intend by this answer to suggest that a candidate other than himself would be the GOP presidential standard-bearer. He reiterated that he had not decided whether he would seek reelection or when he would make the announcement of his decision.

"Well, I suppose sometime next year it has to be done, the decision has to be made," Reagan said. "I think to do so earlier than necessary then opens you to the charge and the suspicion, and certainly the charge from the other side of the aisle, that everything you are trying to do is based on politics, and it also tempts some of your people to base their advice on what they think might affect the next election."

Reagan, who will be 72 in February, does not seem to be in any hurry to decide. His focus at midterm appears to be on the combat with Congress which seems certain to continue in 1983.

"There is a little bit of impatience at what's going on, some frustration," Deaver said. "The Washington system frustrates him, as the Sacramento system did."

Another aide said that Reagan is "much more aware of the time that's left in his presidency" and is going through a process of sorting out his essential priorities.

"It's very clear to him that he's not going to accomplish everything he set out to do," this White House official added.

Reagan displayed some of this impatience in the interview. He repeatedly jabbed at Congress on defense and economic issues. Reading from a memo prepared by his staff, he gave numerous examples of a Democratic jobs program he termed "a pork barrel in the old-fashioned sense."

Except for this issue, however, Reagan talked extemporaneously, demonstrating a familiarity with issues that eluded him in some earlier interviews. Aides said the president is sensitive about published accounts, in The Post and other newspapers, that he is ill-informed and out-of-touch on some key questions.

In a nationally broadcast interview yesterday with six radio correspondents, Reagan sounded defensive when asked whether he knows what it is like to be unemployed. As president, "you are not that separated from the world," Reagan said. He mentioned his daily contact with aides, with security guards and on the campaign trail as keeping him in touch, and recalled his own experience seeking work in the Great Depression in 1932. And he added, "When I go to the ranch, sometime out there I'm right back with the neighbors and the people that work there. And it is as if this had never happened."

In Thursday's interview with The Post, Reagan, who had been briefed for 20 minutes in advance, struck an informed and reflective tone. Only at the end, and lightly, did he lapse into the set-piece rhetoric of the recent political campaign, saying, "Could I coin a term and say, stay the course?"

Otherwise, Reagan did not employ the one-liners that are his trademark. He was serious in manner and tone, reflecting the sober realities that now envelop him and his administration.

While Reagan has aged in office less conspicuously than many of his predecessors, the wear and tear of the presidency is beginning to show in small ways. His once jet-black hair is now streaked with gray. He hears less well than he used to. He also chafes at the luxurious confinement of the White House and takes every opportunity to get away to his California ranch or to go horseback riding in Rock Creek Park, as he did Friday.

"You kind of live like a bird in a gilded cage," Reagan said in the radio interview yesterday, "and I sometimes look out the window at Pennsylvania Avenue and wonder what it would be like to be able to just walk down the street to the corner drug store and look at the magazines. I can't do that anymore."

His aides go so far as to describe Reagan physically as something of a marvel for his age. He works out for 20 minutes or more each day, using an exercise machine. He watches his diet, and his weight.

He also watches Congress. During the lame-duck session he has developed the habit of watching C-span, a closed-circuit television broadcast of congressional proceedings. If a congressmen says something striking, Reagan occasionally will call him up and tell him what he thinks.

As the president's stunning congressional victories of 1981 have receded into the distance, Reagan has grown more and more accustomed to taking half a loaf or less from the Congress and celebrating it as a victory. Looking ahead, he also has recognized new limits on what he can accomplish as president.

In the Post interview, Reagan acknowledged that budget cutting is not the easy task he portrayed it to be during the presidential campaign. Back then, he talked about "waste, fraud and abuse" as if they were line items in the federal budget, which he once proposed to balance by the current fiscal year.

Instead, Reagan in recent weeks has presided over the writing of a budget expected to include a $155 billion deficit. Speaking of the difficulty of cutting domestic social spending, Reagan said, "Many of these programs, they've structurally built the deficit in."

Reagan came into office believing he could hold the allegiance of blue-collar voters with a strategy for economic growth that would create jobs. Instead, unemployment has reached the highest point since 1941, and Reagan acknowledges that it is a more stubborn problem with dimensions that he had not envisioned.

The change in Reagan is most striking in foreign policy. He knew relatively little about foreign affairs when he took office, and his administration was focused on domestic economic policy. The tension between the president and his first secretary of state, Alexander M. Haig Jr., made him reluctant to venture into the personally uncharted waters of foreign policy.

But this has changed since George P. Shultz succeeded Haig last summer. Reagan is comfortable in his relationships with the easygoing Shultz and with longtime confidant William P. Clark, his national security adviser. Both men have seen to it that Reagan is consistently briefed on key foreign policy issues, and Reagan has gained confidence as he has become more knowledgeable on foreign affairs.

This confidence showed in a recent briefing of ambassador Philip C. Habib before he left on his latest round of Middle East talks. According to administration officials, Reagan took an unaccustomed leading role, emphasizing to Habib that he thought the withdrawal of Israeli troops from Lebanon was proceeding too slowly.

Reagan made a similar point publicly in the interview, saying "the time is now for action" on withdrawal of all foreign troops from Lebanon.

Discussing the Soviet Union, Reagan took a broader and more complicated view of events than he once did. He was careful not to directly criticize Yuri Andropov, the new Soviet leader, but said "there is a limit to how far any man in his position can go without the support of the Politburo."

In the past, Reagan often has talked as though only the impact of western military might could alter Soviet behavior. Without backing down on his defense buildup, Reagan suggested in the interview that the Soviets might also be influenced by the deterioration of their economy and the need to produce more consumer goods.

Whether Reagan's fundamental policies will be changed by his growing awareness of the complex nature of the world is not yet clear. For all the impact of the presidency upon him, Reagan strikes his longtime associates as being much the same person he has always been, believing in the same verities.

But the path to Reagan's major goals -- restoration of U.S. stature in the world, economic recovery and a smaller federal government -- no longer seems well-marked.

Even aides who view Reagan as essentially unchanged from his early days in politics can see shades of gray emerging from the shaping experience of the presidency.

"Of course, it's changed him," Deaver said. "It would change anybody."