The extraordinary thing about the Soviet Union is the way it changes over the years while at the same time remaining the same.

This may sound like a contradiction, but a traveler returning to Moscow after a long absence cannot otherwise bring order to the conflicting assaults on his senses.

The other evening, an old friend was telling me about his life. When I last saw him eight years ago, he was passionately in love with a girl from Finland. They wanted to marry, but he could not get permission.

At that time, he was employed in an important state institution. When he told his superior of his plans, he was given a clear choice - either keep his job or marry a foreigner, but not both. He chose the girl and lost the job.

This sounds like the beginning of one of a story to capture Hollywood's imagination, except that it was real and simple. She traveled as a tourist from Helsinki to Leningrad, but was not allowed to travel to Moscow. Hence he had to go to Leningrad for a few days of passion and long months of waiting for permission to marry.

I has missed the end of his tale, when my assignment ended and now, as we walked in the woods on a balmy August evening, he supplied the details. After 2 1/2 years of waiting, he said, both of them became desperate. He could see no future in the affair, nor could she. Yet they were deeply in love, he said describing their traumatic parting in Leningrad.

She had since married a Finn and he married a Russian woman.But then, he added wistfully, had they only been able to hold out a few years longer, the whole thing would have worked out.

"Today it would be possible. Today you can get permission to marry a foreigner," he said. "Things are changing after all."

ONE THING that immediately surprises the senses here is the variety of colors in the streets. Ten years ago the grayness and shabbiness were dominant. Soviet women are no longer light years behind Western fashion. One comes frequently upon smartly dressed women who look as if they were dressed in a top Western European fashion store.

Marching in Red Square one morning was a group of Pioneers, each wearing the obligatory red scarf. Yet they also wore jeans.

The Soviet state has abandoned its once unrelenting resistance to things Western. Since Scotch whiskey is a new fad among urban Russians, the state is producing its own brand. It is simply labeled viski and costs $5 a fifth.

There is also a growing number of private automobiles, and although police checkpoints are still on virtually ever major intersection, it is much more difficult to keep track of who is going where.

And while the massive police apparatus remains in place, one has the impression that contacts with Soviet citizens have become easier and more relaxed.

One Sunday afternoon we drove out to what is known as the "diplomatic beach" near the village of Uspenskoe, about 15 miles outside the city limits.

When I first worked here in the late 1960s, the muddy beach on the banks of the Moscow River was the only place we could go to outside Moscow without special permission. You took the scenic "government road" called that because only officials and foreigners had ready access to it. Passed several police checkpoints and string of small villages. On the left side, roughly half way to Uspenskoe, stands Stalin's dacha surrounded by a high brick wall and empty since the dictator's death.

That was a melancholy time, not so much because of the restrictions of foreigners but because of the restrictions on Soviet citizens. "Our" beach was off limits for Russians, and so were we, almost as if we collectively carried the germs of a deadly disease.

Ten years ago was also the invasion of Czechoslovakia, which touched off a momentous propaganda battle and produced a widespread depression here that almost blighted life. So we drove to Uspenskoe then in search of country air and away from the long-striding cops who would come lunging in whenever we tried to speak to Muscovites in streets and squares.

"The "diplomatic beach" is still used by foreigners, but this time it seemed as though the Russians outnumbered us here. Moreover, Uspenskoe is not the only place you can go these days without permission. There are 12 towns in the Moscow region open to foreigners and countless other places for outings and picnics.

ALL THIS may sound like small changes in life style, cosmetics hiding the warts. Perhaps. But those in the young generation seem unlike their parents. Fear lingers on, but it is not the kind of fear that stifles expression.

Less than a week after my arrival in Moscow, I found myself dining in the home of a Soviet journalist. It was an evening of good conversation, good food and excellent wine. The host was considerate and not given to propagandistic outbursts.

This invitation, apparently spontaneous and without an ulterior motive, would be of a little significance if I had not spend 2 1/2 years here a decade ago without ever once having been invited to the home of a Soviet journalist.

I spoke with another Soviet acquaintance about the unexpected dinner.

"You see," he said, somewhat embarrassed, "about five years ago the strict prohibition on meetings with foreigners was lifted. Despite that, many people are afraid, since it could be used against them and damage their career. But some are not afraid. That's all."

What he suggested was not that mingling with Americans or other foreigners is encouraged, there are no longer strict rules prohibiting such contacts.

AND YET there is about Moscow an air of inertia suggesting that nothing has changed, at least nothing basic in the way the country and its economy are run. The men in the Kremlin have been in power for a long time and perhaps it is illusory to expect any sharp policy departures from the old generation.

The government seems to have begun to heed popular demands for improved living standards. Further, more consumer goods are available today than were a decade ago. But after decades of austerity, the demand is so high that this centrally planned economy simply could not meet it. Hence there are grumbles, a black market and long lines.

The shops do have a greater variety of gods, yet this continues to be an economy of shortages. Much of what is produced is shody or obsolete. The shops, to get rid of such items, force them on customers.

If you wanted to buy fresh oranges at the market the other day, you had to buy one pound of rotten apples for each two pounds of oranges. A man from the provinces came to Moscow to buy a rare book, found it in a bookstore but could have it only if he purchased 20 volumes of Communist Party literature as well.

Lipsticks, mascara and other cosmetics are virtually unobtainable. An enterprising women and her daughter set upp a stall in an underpass in central Moscow selling their homemade lipstick, eyebrow pencils and artificial eyelashes a few days ago. The customers, on opening their purchases, detecteed a strange smell. It turned out that the base materials were shoe polish and cheap perfume. The two were arrested.

Inefficiency, waste and corruption are an old story. And so is the government's effort to create the best impression, to hide the warts and always to put the best foot forward.

The other day, Deputy Foreign Minister Anatoly Kovalev was holding a press conference in the brand new building of the Tass news agency. At one point he was asked about the U.S. cancellation of the sale of a Sperry Rand computer for Tass.

Dismissing the cancellation as unimportant, Kovalev said Tass would continue to function efficiently and provide good service to its customers.

It was at this point in the Tass dispatch on its English wire that the Tass machine broke down, and was unable to complete the story for several minutes.